Nanoanalises

Prévia do material em texto

Nano Electronics: A New Era of Devices 
Inderpreet Kaura, Shriniwas Yadavb , Sukhbir Singhc , Vanish Kumard, 
Shweta Arorae, Deepika Bhatnagarf 
Biomolecular Electronics and Nanotechnology Division (BEND), Central Scientific Instruments 
Organization (CSIR-CSIO), Sector-30C, Chandigarh, 160030, India. 
ainderpreety@yahoo.co.in (corresponding author), bsniwas89@gmail.com , 
csinghsukhbir05@gmail.com, dvanish.saini01@gmail.com , earorashweta_pharma@yahoo.co.in, 
fdeepikabhatnagar8@gmail.com, 
 
Key words: Nanoelectronics, Molecular electronics, Nanogap fabrication, Graphene, CNTs, Band 
gap, HOMO-LUMO, CNT-FET 
 
Abstract: The technical and economic growth of the twentieth century was marked by evolution of 
electronic devices and gadgets. The day-to-day lifestyle has been significantly affected by the 
advancement in communication systems, information systems and consumer electronics. The 
lifeline of progress has been the invention of the transistor and its dynamic up-gradation. Discovery 
of fabricating Integrated Circuits (IC’s) revolutionized the concept of electronic circuits. With 
advent of time the size of components decreased, which led to increase in component density. This 
trend of decreasing device size and denser integrated circuits is being limited by the current 
lithography techniques. Non-uniformity of doping, quantum mechanical tunneling of electrons from 
source to drain and leakage of electrons through gate oxide limit scaling down of devices. Heat 
dissipation and capacitive coupling between circuit components becomes significant with 
decreasing size of the components. Along with the intrinsic technical limitations, downscaling of 
devices to nanometer sizes leads to a change in the physical mechanisms controlling the charge 
propagation. To deal with this constraint, the search is on to look around for alternative materials 
for electronic device application and new methods for electronic device fabrication. Such material is 
comprised of organic molecules, proteins, carbon materials, DNA and the list is endless which can 
be grown in the laboratory. Many molecules show interesting electronic properties, which make 
them probable candidates for electronic device applications. The challenge is to interpret their 
electronic properties at nanoscale so as to exploit them for use in new generation electronic devices. 
Need to trim downsize and have a higher component density have ushered us into an era of nano-
electronics. 
 
Contents of Paper 
 
1. Introduction 
2. Molecular Electronics 
2.1. Potential Organic Molecules That Mimic the Traditional Semiconductor Electronic 
Components 
2.1.1. Molecular Wires 
2.1.2. Molecular Resistor 
2.1.3. Molecular Diodes 
2.2. Reason for Molecules as Electronic Components 
2.3. Recognizing the Components of the Molecules in the Circuit 
3. Metal –Molecule-Metal Junctions 
3.1. Methods for Fabricating Nanogap Electrodes 
3.1.1. Mechanical Controllable Break Junctions (MCB) 
3.1.2. Angle Shadow Evaporation And Shadow Masking 
3.1.3. Scanning Probe Lithography (SPL) 
3.1.4. Electron Beam Lithography 
Solid State Phenomena Vol. 222 (2015) pp 99-116
© (2015) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/SSP.222.99
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,
www.ttp.net. (ID: 115.248.234.162-05/11/14,07:03:32)
http://www.ttp.net
3.1.5. Electromigration 
3.2. Nano Architectures and Arrays 
4. Carbon Electronics 
4.1. Carbon Nanotubes based Electronics 
4.1.1. Carbon Nanotube based FET 
4.2. Graphene based Electronics 
4.2.1. Band gap Modulation and Graphene devices 
5. Theoretical point of view and quantum phenomena 
5.1. A Fundamental Approach to Electron Transport 
5.1.1. The Electron Transfer theory 
5.1.2. Landauer Formula and Electron Transport through Molecular Junction 
5.2. The Relationship between Electron Transfer Rates and Molecular Conduction 
5.3. A Frontier Molecular Orbital Approach to Electron Transport 
5.3.1. Forward-bias condition 
5.3.2. Reverse-bias condition 
6. Conclusions 
References 
 
1. Introduction 
The invention of the transistor in 1947 is one of the most important inventions of the 20th century. 
Since its inception, we have witnessed dramatic advances in electronics that have found uses in 
computing, communications, automation and other applications that affect just about every aspect 
of our lives. To a large extent these advances have been the result of the continuous miniaturization 
or ‘scaling’ of electronic devices, particularly of silicon-based transistors, that has led to denser, 
faster and more power-efficient circuitry. Unfortunately, the scaling down must eventually end. 
Increasing power, capital costs, and ultimately theoretical size limitations, are poised to halt the 
process of continually shrinking the transistor. The realization of the approaching limits has inspired 
a worldwide effort to develop alternative device technologies. Some approaches involve moving 
away from traditional electron transport-based electronics: for example, the development of spin-
based devices. Another approach, maintains the operating principles of the currently used devices 
primarily that of the field effect transistor, but replaces a key component of the device, the 
conducting channel, with alternate material which have superior electrical properties. Taking into 
consideration the second approach, inexpensive, functional and atomically precise molecules could 
be the basis of future electronic devices, but integrating them into circuits will require the 
development of new ways to control the interface between molecules and electrodes. 
 
Molecular-electronics show promise as a technology to continue the miniaturization of ICs. 
However, whether molecular-electronics will be a replacement for conventional ICs, or as a 
complimentary technology, is yet to be determined. What has already been shown is that 
components such as wires and molecular switches can be fabricated and integrated into 
architectures. It is also known that these devices will be prone to defects, fluctuations and that fault 
tolerance schemes will be an integral part of any architecture. 
 
The greatest progress has been made in the research of the components that may make up nano-
electronics. Researchers have been able to fabricate molecules that have two states, such that the 
molecules can be switched “on” and “off”. Some of these molecules have shown the functionality 
of diodes or variable resistors. Scientists have also been able to fabricate silicon nanowires and 
carbon nanomaterials such as one-dimensional (1D) carbon nanotubes (CNT) or two-
dimensional (2D) graphene layers. Both of these technologies can be used as wires or devices, and 
in some cases both. Nanoimprint lithography, probably the most promising wire fabrication 
technique, has been used to produce working memories on the nanometer scale. While all of these 
100 Nanomaterials
devices have been demonstrated, more lot of research is required to reliably produce such 
analogues. 
 
2. Molecular Electronics 
The field of molecular electronics has been around for more than 40 years, but only recently has 
some fundamental problems been overcome. It is now time for researchers to move beyond simple 
descriptions of charge transport and explore the numerous intrinsic features of molecules. 
Fundamentals of electronics say that all the electronic operations take place through the transport of 
the electron in the circuit. Robert Mulliken and Albert Szent-Gyorgi in 1940 [1] advanced the 
theory of molecular conduction and did an interesting study of charge transfer in "donor-acceptor" 
systems. Bringing out correlation in such donor-acceptor systems where the charge transfer can be 
achieved easily suggested these systems would be suitable for the molecular electronic devices[2]. 
 
In the early 1970s, a visionary concept of exploiting the intrinsic functionality of molecules for 
electronics was sketched out by Arieh Aviram and Mark Ratner [3]. In their pioneering theoretical 
work, Aviram and Ratner suggested that a single molecule (Fig. 1) could function as a rectifier. The 
molecule would mimic a semiconductor-like band structure by taking advantage of electron-rich 
and electron-poor moieties to achieve one-way conduction through differently aligned molecular 
orbitals with respect to the Fermi energy of the electrodes [2, 3]. 
 
With this excellent article by Aviram-Ratner the era of molecular electronics was established. 
 
 
Fig. 1: Proposed molecular rectifier by Aviram and Ratner [3]. 
 
2.1. Potential Organic Molecules that Mimic the Traditional Semiconductor Electronic 
Components: Traditional electronics has many components like conductor (wires), resistor 
(insulating connection), diode (rectifier), transistor (triode), logic circuits, etc. [4]. Among all these, 
the most fundamental components are wire, resistor and rectifier, which are discussed below. 
Transistor is also having equal importance in the field of electronics, but it can be easily fabricated 
from the diode by utilizing a suitable doping of a gate electrode. 
 
2.1.1. Molecular Wires: Electronic wires are the components through which electric current can 
pass from one end to the other end freely. Organic molecules, which can mimic the wire function, 
are the π-type systems as shown in fig. 2. 
 
Fig. 2: Potential organic molecular wires. 
 
In these molecules, the process of electron transfer takes place through the backbone of fully 
delocalized π-bridges, and consequently energetically closely spaced frontier molecular orbitals 
Solid State Phenomena Vol. 222 101
(reduced HOMO-LUMO gap or in short, HLG) are the conduction channels. Due to very small 
HLG, the process is thermodynamically favorable and ultimately gives rise to efficient wire 
function. 
 
2.1.2. Molecular Resistor: Organic molecules to achieve resistor type of behavior are as shown 
below in Fig. 3. 
 
Fig. 3: Potential organic molecular resistors. 
 
In these types of molecules, the presence of the saturated –CH2 - units creates nodes in their electron 
densities above the atomic nuclei. For this reason and also due to large HLG, they cannot transport 
electrical current. This enables aliphatic molecules or groups to act like resistors. 
 
2.1.3. Molecular Diodes: Starting from the AR rectifier [3] to till date, the common construction 
principle of organic molecular rectifier adopted is as shown in Fig. 4. A comparison of the AR 
rectifier (Fig.1) with that of the semiconductor diode will give a clear idea about the rectification 
ability of general organic molecular rectifiers. 
 
Fig. 4: Potential organic molecular rectifiers. 
 
A structural correlation of the AR rectifier to a normal silicon junction diode shows that, acceptor 
part of the molecule can be mimicked with p-type semiconductor, donor part can be mimicked with 
n-type semiconductor and σ-bond can mimic the pn-junction barrier. With these favorable structural 
features of an organic molecule, it can be expected to result in similar characteristics like that of a 
semiconductor rectifier. 
 
2.2. Reason for Molecules as Electronic Components: The distinguished points, which can be 
put in favor of molecules as electronic components are: 
(1) Molecules are of very small size. A molecule is around few thousand times smaller in size than 
that of the presently used semiconductor transistor. 
(2) In semiconductor devices, due to the band structure, electron can stay at any level of the band, 
which can probably interfere with other devices. In the molecule, the energy levels are quantized 
and discrete and hence the interference can be nullified. 
(3) Due to delocalizable π-systems present in the molecule, the electron transport will be 
thermodynamically more favorable compared to the semiconductor systems. 
(4) Due to the flexible nature of the molecule (especially in π-systems due to cis- & trans-
isomerism) switching function (on and off control) can be easily achieved by the simple alternation 
of the two conformations. 
(5) Due to exact chemical equivalence of the molecules, it can be fabricated in a defect free fashion. 
(6) Another important property of organic molecules is its self-assembling nature, which will be 
helpful in manufacturing large arrays of identical devices. 
 
2.3. Recognizing the Components of the Molecules in the Circuit: There are some 
fundamental questions which can arise in one’s mind that how molecules can be interleaved in an 
electronic circuit. The various widely used methods (for inserting a molecule in an electronic 
102 Nanomaterials
circuit) include scanning tunneling microscopy (STM), conducting atomic force microscopy 
(AFM), break junctions, fixed-gap nanojunctions, nanopores, mercury drop contacts and crosswire 
assemblies. All the methods discussed above have their advantages, but the major difficulty in all 
this is counting the number of contacts molecules and the characterization of their bonding patterns. 
“Tour-de-force assembly mechanism” for interleaving a molecule into circuit is one such 
outstanding method [4, 5]. 
For a better understanding of the circuital operation of the inserted molecule into the nano-junction, 
a schematic diagram is shown in Fig. 5. 
 
 
 
Fig. 5: Schematic representation of the circuital operation of an organic (a) Molecular wire, and (b) 
Molecular rectifier. 
 
Though the actual measurement process is not so simple, the above circuit diagram presents the 
essential components and the background operation principles of the molecular wire or rectifier in a 
simpler way. The diagram shows that, besides the external power supply and the output 
measurement components, there are three most important components, which are essential in the 
device fabrication as described below: 
 
The Molecule: Here the molecule is either the wire or the rectifier. In both the cases, the molecule 
is needed to be both ends selected in order to make contact with the electrodes. 
 
Gold contacts: There are two gold contacts; left contact is named as Cathode and right contact as 
Anode. These create the connection between the molecule and the external power supply. 
 
Solid State Phenomena Vol. 222 103
Alligator Clip: The two thiol groups present in the molecules are known as alligator clips and the 
name comes as it clips the molecule to the two electrode contacts. It serves the channel between the 
electrodes and the molecule. 
 
In a simplified way, the operation can be explained as the alligator clips present in the molecule, 
hook up the molecule to the two metal nano-electrodes and these two electrodes are connected to 
the external circuit. Once a bias voltage is applied in the external circuit, one electron from the left 
(by convention) electrode will be loaded to the molecule through the metal-molecule junction and 
travels through the molecule to escape to the right electrode through the right hand side metal-
electrode junction. During this process of the electron transport, a current will be realized in the 
outer circuit. 
 
3. Metal –Molecule-Metal Junctions 
The ultimate aim of molecular scale electronics or single molecule electronics is to use the 
individual molecule or atom as a functional component. The struggle to control the position and 
distance of smaller and smaller numbers of atoms in the active regions of devices can be made 
using top-down methods. However, already in the earliest experiments, the vision of molecular 
electronics encountered tremendous difficulties. First, and in contrast to silicon where research was 
facilitated by the availability of large crystals, the size of an individual molecule cannot be easily 
scaled up, which means that atomic-sized electrodes areneeded to contact an individual molecule. 
By pulling and then breaking ductile metal wires, suitable electrodes can be fabricated, and over the 
past 15 years a variety of innovative approaches have been developed to experimentally conduct 
charge-transport studies at the few molecular levels. Second, when the number of active molecules 
in the junction was reduced down to a single molecule, the variability of the ‘devices’ increased 
because the molecular junction became sensitive to every microscopic detail of its atomistic 
configuration [6]. Until now, only a few experiments have gained control over the crucial atoms in 
the junction, but such atomic control is essential for the development of molecular electronic 
applications. 
 
This can be achieved by miniaturizing electrical circuits and connections for device functioning. 
Several approaches have been used to provide electrical connections to molecules and cluster of 
molecules. Nanogap electrodes are a pair of electrodes separated by a nanometer gap to form metal-
molecule-metal junctions for practical device fabrication. 
 
3.1. Methods for fabricating nanogap Electrodes: Precise control of spacing makes it more 
difficult and challenging because it goes beyond the capability of micro-fabrication techniques like 
photolithography. In the last few years, several effective and creative methods for nanogap 
electrode fabrication have been reported, including mechanically controllable break junctions, angle 
shadow evaporation and shadow masking [5], scanning probe lithography [7], electron beam 
lithography [8], and electro-migration [9]. 
 
3.1.1. Mechanical controllable break junctions (MCB): A mechanical controllable break 
(MCB) junction was first introduced by Moreland and his co-workers from the US National Bureau 
of Standards to form an electron tunneling junction [10]. This technique was then adopted by 
various researchers to create nanogap junctions with several nanometer separations. In this method 
a notched metallic wire is glued over an elastic substrate, which works as the bending beam. The 
substrate is bent by pushing its center with a driving rod to fracture the notched wire, after which an 
adjustable tunneling gap can be established. The breaking process is mostly conducted under low 
temperature and high vacuum conditions to avoid contamination. Although the MCB junction 
method was useful for fundamental investigation, it was not facile to fabricate highly integrated 
molecular devices and also difficult to controllably fabricate relatively large gaps. 
 
104 Nanomaterials
3.1.2. Angle shadow evaporation and shadow masking: The angle shadow evaporation method 
was established by Dolan in 1977 [11, 12]. This technique was then accepted and refined for the 
fabrication of nanogap electrodes. Shadow evaporation is often combined with optical and electron-
beam lithography to define metal leads [13]. The gap size can be adjusted by changing stepwise the 
tilt angle until a desired space is obtained. Some researchers have also utilized physical shadowing 
of nanoscale objects like carbon nanotubes for creation of nanogap electrodes. Sun et al. [14] 
achieved 3 nm gap electrodes and applied them to the electrical study of nanocrystals successfully. 
 
3.1.3. Scanning probe lithography (SPL): This method is also known as dip pen nanolithography 
used in nanofabrication developed by Zhang et al. [16] in Northwestern University. In this 
technique an atomic force microscope tip is used to pattern directly on a range of substances with a 
variety of inks. There are two basic types of SPL. One is a destructive method that physically, 
chemically, or electronically deforms the substrate’s surface to make up the pattern [7]. The other is 
a constructive way in which the patterning is done by directly transferring chemical species to the 
surface [16]. The main limitation of this technique was the damage of the probe and unsatisfactory 
material removal due to a sharp tip. 
 
3.1.4. Electron beam lithography: Electron beam lithography (EBL) is quite efficient and 
controllable technique for nanoscale fabrication. In this technique a highly energized and focused 
electron beam is used to create nanopatterns over resist coated substrate. EBL is generally used to 
firstly realize nanogap electrodes with space at 10–20 nm [13], and then other techniques, such as 
electro deposition, shadow mask evaporation, are implemented to further narrow the gap width to a 
1-5nm scale. The major drawback of this method involves back scattering of highly energetic 
electrons, which affects the feature size. 
 
3.1.5. Electromigration: Electromigration is a thermally assisted process where nanogap is 
created by high current density to create nanogap in thin metal substrates by joule heating. 
However, excessive heating should be avoided because it would cause undesired melting of the 
metal. More recently, it has been well utilized to fabricate nanogap electrodes for nanodevices. 
Recently, researchers have also performed electromigration in carbon nanotubes and graphene for 
nanogap electrode fabrication [17, 18]. Compared with thin metal wires (20 nm) that could only be 
prepared by electron-beam lithography, carbon nanotubes possess high conductivity, more 
favorable configurations, and better contacts to organic molecules by C-C bonding; thus they are 
considered good substitutes for metal wires to fabricate nanogap electrodes. 
 
3.2. Nano Architectures and Arrays: One of the big questions for the future nano-electronics is 
whether nano-scale devices can be reliably assembled into architectures. Some small-scale 
successes have been achieved, but to get the benefit of nano-electronics the enormous integration 
levels may be desired. The most promising architectures to date are array based. This is because 
arrays have a regular structure which is easier to build with self-assembly. Arrays also make good 
use of the available devices (nanowires, carbon nanotubes, and molecular electronics), and they are 
easy to configure in the presence of defects. There are other more random architectures that would 
require even less stringent fabrication techniques, but there is some doubt about how they will scale 
to larger systems. Overall, it is difficult to evaluate architectures as the underlying components are 
not fully understood nor developed yet. One thing that seems clear is that nano-electronics will, at 
least for the first few generations, need the support of conventional lithography based electronics 
for things such as I/O, fault tolerance, and even simple signal restoration. 
 
Fault tolerance is another big problem for nano-electronics. It seems evident that the manufacturing 
techniques may never be able to produce defect free chips, so fault tolerance will be key to the 
success of nano-electronics. For manufacturing defects, detecting and configuring around the 
defects is the most economical technique, since nano-electonics will be configurable devices. The 
Solid State Phenomena Vol. 222 105
hard problems are detecting the defects among 1012 devices in an economical manner, and how to 
best manage the large defect map. It also appears that transient faults will be a problem with nano-
electronics due to their small size and low current levels. To handle transient faults, a hardware 
redundancy method such as multiplexing or NMR will have to be used to dynamically detect and 
repair faults. Unfortunately, these methods would require too much redundancy to handle the 
number of manufacturing defects expected [6]. 
 
4. Carbon Electronics 
Apart from organic molecules, there are so many potential molecules available which can be 
explored for molecular electronics, e.g. Biomolecules: DNA, Proteins, and Carbon materials: 
Carbon nanotubes, Graphene, etc. Since biomolecules are highly unpredictable in the environmental 
conditions hence DNA and protein’s electron transport is necessary to understand life processes butare not sufficient for nanoelectronics. Rather, carbon materials having intriguing electrical 
properties can be a very good future option for nanoelectronics. 
 
4.1. Carbon Nanotubes based Electronics: Carbon Nanotubes (CNTs) are allotropes of carbon 
with cylindrical shapes and very high length to diameter ratio of the order of 1,32,000,000:1 [19]. 
CNTs are of hollow structure formed by rolling, one atom thick sheet of sp2 hybridized carbon 
atoms named as graphene. CNTs are characterized as single wall CNTs (SWCNTs) and multiwall 
CNTs (MWCNTs). Electronic properties of CNTs depend upon its chiral angle (angle along which 
graphene sheet is rolled) and radius of nanotube. 
 
 
(a) (b) 
Fig. 6: (a) The (n, m) nanotube naming scheme can be thought of as a vector (Ch) in an infinite 
graphene sheet that describes how to "roll up" the graphene sheet to make the nanotube. T denotes 
the tube axis, and a1 and a2 are the unit vectors of graphene in real space, and (b) Different types of 
CNTs. 
 
The structure of a SWNT can be conceptualized by wrapping planar sheet of graphene into a 
seamless cylinder as shown in fig. 6. The way, the graphene sheet is wrapped is represented by a 
pair of indices (n, m). The integers, n and m, denote the number of unit vectors along two directions 
in the honeycomb crystal lattice of graphene. If m = 0, the nanotubes are called zigzag nanotubes, 
and if n = m, the nanotubes are called armchair nanotubes. Otherwise, they are called chiral 
nanotubes. Diameter of ideal CNT can be calculated from its unit vector (n, m) as follows: 
 
D (in pm) = 78.3(n2+m2+nm)0.5 
All MWCNTs are of metallic in nature but nature of SWCNTs varies with chiral vectors n and m. 
SWCNTs are very important type of CNTs, because their properties vary significantly with chiral 
vectors. The band gap of SWCNTs can vary from 0 to 2eV, thus electrical properties can be 
metallic as well as semiconducting. A major problem in synthesis of SWCNTs is the lack of 
106 Nanomaterials
synthetic methods that yield exclusively semiconducting nanotubes, which has stimulated numerous 
attempts to either separate semiconducting tubes from the as-prepared material or to selectively 
eliminate the metallic tubes [20]. The separation approach has mainly depended on non-covalent 
chemical functionalization by various types of polymers capable of selectively wrapping 
semiconducting SWCNTs, most conspicuously polyfluorenes [21]. Another method based upon the 
selective binding of semiconducting tubes by the terminal amino groups of the silane layer on the 
silica has been explored for self-sorting of SWCNTs by spin-coating nanotubes from solution onto 
appropriately surface-functionalized Si/SiO2 substrates [22, 23]. Some other efficient chemical 
methods including effect of dizonium salt and plasma ion etching to eliminate metallic nanotubes 
are also used [24]. 
 
4.1.1. Carbon Nanotube based Field Effect Transistor: Carbon Nanotube based Field Effect 
Transistor (FET) (Fig. 7) functions as Schottky barrier transistor rather than conventional bulk 
transistors. In the conventional transistors the gate bias not only affects the carrier density through 
conducting channel but also the transmission through junctions [25]. Hence, both junction potential 
as well as carrier potential was affected by gate potential. That is why, to minimize Schottky barrier 
for one type of charge carrier, proper contact metal choice is needed. It was first demonstrated for 
palladium contacts enabling nearly barrier-free access to the valence band of semiconducting tubes 
[26]. Schottky barrier potential can be reduced by selectively doping contacts of carbon nanotubes 
which has been realized on the basis of complex charge transfer between CNTs and adsorbed 
molecules. In order to optimize the gate switching, the capacitive coupling of the gate electrode has 
to be enhanced. In the ideal case, the classical electrostatic capacitance Cg would become larger 
than the quantum capacitance Cq of the tube (Cq= 10-16 F/µm), and therefore, dominates the 
switching action [27, 28]. 
 
Fig. 7: CNT-FET having patterned drain and source contacts of Au/Ti over CNT. 
 
A promising approach takes advantage of the excellent insulating capability of high-quality organic 
self-assembled monolayers in combination with a thin, oxygen-plasma-grown oxide layer for strong 
gate coupling to CNTs. It has been reported that SWCNT-FET using SiO2 with silane, showed 
excellent operating voltage of 1V and sub-threshold swing of 60mV per decade (A decade 
corresponds to a 10 times increase of the drain current Id) [29]. Significant progress has also been 
achieved in the development of FETs incorporating highly ordered SWCNT arrays produced via 
oriented CVD growth on quartz substrates. Remarkably, even without enrichment of 
Solid State Phenomena Vol. 222 107
semiconducting tubes, the transistors display excellent performance, as reflected in sub-threshold 
swings as low as 140mV per decade, mobility of up to 80cm2/Vsec., and operation voltages below 5 
V. A key factor to achieve this has been to reduce the probability of metallic pathways through use 
of sufficiently narrow network stripes. 
 
The major disadvantage of CNTs, however, is their random distribution, which clearly hampers 
their utilization as a replacement for silicon as a substrate. This leaves two options for carbon-
electronics: either self-organization methods for CNTs or carbon “substrates”, thin layers with 
similar properties to CNTs. 
 
High on the list is graphene, planar sheets of honeycomb carbon rings just one atom thick. This 
nanomaterial supports a range of properties including ultra-strength, transparency (because of its 
thinness) and blisteringly fast electron conductivity—that make it promising for flexible displays 
and super speedy electronics. 
 
4.2. Graphene Based Electronics: The impressive physical properties of graphene like unique 
optical transparency, superior mechanical strength, and excellent charge carrier mobility make it a 
suitable candidate for device applications which include electronics, optoelectronics, photonics, and 
spintronics. The most significant electrical property of graphene is due to the presence of massless, 
chiral, Dirac fermions which manifest as high carrier mobility of the order of 10,000 cm2/Vs in 
experimental measurement [30] and a theoretically 27,000 cm2/Vs [31]. Therefore it should enable 
transistors of very high frequency. 
 
Isolated only four years ago, graphene already appears in prototype transistors, memories and other 
devices. Graphene is an extremely promising material in the field of electronics. It comes majorly in 
two variants: Zigzag and Arm chair, depending on the edge pattern as visible in fig. 8. However, 
when using it as a material for transistor channel without any further improvement, the insufficient 
on/off ratio has been pointed out due to inadequate band gap. Nevertheless, many potential 
solutions to band gap formation have been proposed, such as application of the vertical electric 
field, forming graphene in a ribbon structure or modulating band gap with chemical functional 
groups. 
 
 
Fig. 8: (a) Graphene sheet with both the zigzag (red) and the arm chair (green) directions, (b) 
AGNRs, and (c) ZGNR [32]. 
108 Nanomaterials
4.2.1. Band gap Modulation and Graphene Devices: Many theoretical and experimental 
approaches have been demonstrated to tailor the graphene electronic structure for opening up 
energy band gap (Eg). The main methods include quantum confinement of charge carriers from 2D 
graphene to 1D graphene nanoribbons (GNR), pi-pi stacking on graphene, chemical doping, and 
application of strain. 
 
Electronic properties of GNR vary with the edge pattern as well as the width of the sheet. Edge 
pattern can be armchair [AGNR] and zigzag graphene nanoribbon [ZGNR] {Fig. 8(b) and (c)}. 
Zigzag GNRspossess metallic characteristics and is independent of width, while there is a lot of 
scope of band gap modulation in AGNRs. These fall into three families, depending on their width. 
With N being number of carbon dimer lines across the ribbon width, these families are N=3p-1, 3p, 
and 3p+1, where “p” is a positive integer. When N=3p and 3p+1, AGNRs have semiconductor-like 
behaviors, while N=3p-1, AGNRs and ZGNRs are quasi-metallic (narrow gap semiconductor). 
 
It was demonstrated that chemical functionalization of semiconducting GNRs with Stone-Wales 
(SW) defects by carboxyl (COOH) groups forms a stable structure. Theoretically it has been 
demonstrated that when the geometrical structure changes, electronic properties of the GNR 
changes significantly. It is further reported that electrical conductivity of the system considerably 
enhanced with the increase of the axial concentration of Stone-Wales Defects Carboxyl Pairs 
(SWDCPs), the system transforms from semiconducting behavior to p-type metallic behavior [33]. 
The simulation work based on DFT, the chemistry of imperfect graphene with a broad class of 
defects such as Stone-Wales (SW) defects [33], bi-vacancies, nitrogen substitution impurities, and 
edges has been reported [34]. Study have also proved an effect of finite width, chemical surface 
bonding such as hydrogenation, chlorination, bromination, halogenation, amidation and many 
surface attachment of aromatic molecules using pi-pi stacking on the electronic properties of the 
graphene. 
 
The electronic transport properties of zigzag graphene nanoribbons (ZGNRs) are also reported with 
two kinds of triangular defects. Abnormal behavior has been reported, if the orientation of the 
triangle is changed to rightward, the current is depressed much and shows negative differential 
resistance behavior [35]. Their findings indicate that defect designs can be an efficient way to tune 
the electronic transport of GNR nanodevices. No of reports are there showing that electronic 
transport properties are sensitive to twisting deformations for semiconductor-type AGNRs, but are 
robust against twisting deformations for quasi-metallic AGNRs and ZGNRs. The electronic 
conduction becomes weaker gradually for moderate-gap semiconductor-type AGNRs, but gets 
stronger for wide-gap semiconductor-type AGNRs when the twisted angle increases to 120o. While 
for quasi-metallic AGNRs and ZGNRs, the electronic conduction is strong and obeys Ohm’s law of 
resistance strictly [34]. 
 
Experimentally, STM measurements are done on GNRs to study the edge states and observe 
localized near defects [36]. The tight binding calculation based on the atomic arrangements 
observed by STM reproduces the observed spatial distributions of the local density of states. The 
symmetry of ZGNRs plays an important role in electron transport behavior. Asymmetric ZGNR 
displays monotonic transport behavior. However, in symmetric ZGNRs systems, negative 
differential resistance (NDR) has been reported in papers [37]. More recently, more instances of 
NDR were observed or predicted in molecular devices of GNR. Since edges and defects are playing 
major role in defining the characteristic behavior of GNR, hence, these can be exploited for using 
them in futuristic nanolelectronic devices. 
 
Some systematic ab initio investigations of the possibility to create a band gap in a few-layer 
graphene (FLG) via a perpendicular electric field are provided by K. Tang et al. [38]. Arbitrarily 
stacked FLG remain semi-metallic, but rhombohedral stacked FLG demonstrates a variable band 
Solid State Phenomena Vol. 222 109
gap. The maximum band gap in ABC stacked FLG decreases with increasing layer number and can 
be fitted by the relationship ∆ max= 1/ (2.378 + 0.521N + 0.03 5 N2) eV [38]. 
 
Further, the first-principle calculations explore a band-gap opening of 300 meV for graphene under 
1% uniaxial tensile mechanical strain. The strained graphene provides an alternative way to 
experimentally tune the band gap of graphene, which would be more efficient and more controllable 
than other methods that are used to open the band gap in graphene. The results suggest that the 
flexible substrate is ready for such a strain process, and Raman spectroscopy can be used as an 
ultrasensitive method to determine the strain [39]. 
 
Hence to open the energy band gap in graphene is to work with graphene nanoribbons. Therefore, to 
modulate its band gap according to device application one can functionalize and apply external 
electric field on graphene surface to tune the behavior of charge carriers. 
 
Recently, it has been reported that graphene based transistors were fabricated. Electrical transport 
experiments showed that, unlike single-walled carbon nanotubes, unzipped CNTs forming GNR, all 
of the sub 10nm were semiconductors and afforded graphene field effect transistors with on-off 
ratios of about 107 at room temperature [40]. Many efforts have been made to increase the 
performance of GFET by doping with other elements such as boron and nitrogen for synthesizing p-
type and n-type graphene. Till now, a few doping methods have been exploited, including 
substitutional doping, electrostatic doping by external field, and charge transfer doping. Whereas, 
these approaches reveal intrinsic drawbacks, such as undesired defect formation, complex 
processing steps, and subtle sensitivity to environment. To increase electron transport properties, 
the thiol and thiolate groups are used at the edges of graphene to form bond with drain source 
electrode. This side substitution enhances the performance of FET according to their electron 
withdrawing ability [41]. 
5. Theoretical point of view and Quantum Phenomena 
5.1. A Fundamental Approach to Electron Transport: To understand the molecular conduction 
we need to understand the electron transfer (ET) in a donor–acceptor system, Landauer formula and 
electron transport through molecular junction, the relationship between electron transfer rates and 
molecular conduction and a frontier molecular orbital approach to electron transport. 
 
5.1.1. The Electron Transfer theory: The electron transfer (ET) in a donor–acceptor system, 
where the charge transfer is from the donor to acceptor can be represented as a non-adiabatic ET 
rate (KD→A) equation as is shown in equation 1, as follows: 
 
KD→A = (2π/ћ) V2Φ (1) 
 
Where V is the effective electronic coupling and Φ is the thermally averaged nuclear vibrational 
Franck-Condon factor. Also the constant ћ is equal to h/2π, where h is the Planck’s constant. 
 
The concept of this ET process is based on the Born-Oppenheimer separability of electronic & 
nuclear motion and works in the domain where the initial electronic state of the system represented 
by a one electron wave function of the donor (D) transforms in a diabatic way, to the final state 
represented by the one electron wave function of the acceptor (A). 
 
5.1.2. Landauer Formula and Electron Transport through Molecular Junction: As discussed 
earlier, in a molecular junction, the two major parts are the electrodes (metals) and the molecule. In 
an applied bias voltage, when an electron is transformed from one electrode (say left electrode) to 
the other electrode (say right), the resultant process creates conductance through the molecule. 
110 Nanomaterials
Under the condition of a thermal equilibrium existing at the metal junction, the Landauer formula 
for the coherent conductance through the junction can be represented by Eq. 2. 
 (2) 
Here g is the conductance, Φ is the applied voltage, T is the transmission through the molecular 
junction, f is the Fermi levels of the metal electrodes and E is the energy variable between the two 
levels. Using the above equation, the current through a molecular junction can be derived by 
considering the product of population term and a scattering probability. 
 
5.2. The Relationship between Electron Transfer Ratesand Molecular Conduction: Eq. 2 has 
the same role like that of Eq. 1 as described earlier. In other words, both the processes are based on 
the tunneling of electron. The equivalent facet of the process taking place in both the phenomena 
can be described as conductance in the former and rate constant in the latter, and also the transport 
process in the former is taking place in an electronic bath whereas in the latter it is in vibronic bath. 
Comparing the electron transport process through a molecular junction with that of the Fig. 5, it can 
be matched up the discrete levels for the molecule with that of the two adiabatic surfaces, i.e., the 
two surfaces (D and A in Fig. 5) can be more or less compared with that of the HOMO and LUMO 
of the molecule, respectively, and the ∆E can be equated with HOMO-LUMO GAP (HLG). Hence, 
electron transport to occur through the molecule, the primary need is that the Fermi energy of the 
electrode (Ef) must lie within the HLG. With a proper applied bias voltage through the junction, the 
two Fermi levels (of the left and right electrode) differ by =φe, i.e., the voltage times electronic 
charge. Once one of the Fermi levels overcomes the molecular energy level, the resonant electron 
transfer occurs, and as a result of this, the conductance can be realized through the molecule. 
 
When the molecule is placed between the metallic contacts, e.g. gold, without applying the bias 
voltage and the contacts are coupled (Chemisorbed or adsorbed) to the base, the electrons flow in 
and out of the device bringing them all in equilibrium with a common electrochemical potential µ. 
In this equilibrium state the average number of electrons in any energy level is typically not an 
integer, but is given by the Fermi function: 
f0 (E- µ) = {1+exp [(E- µ)/KBT]}-1 (3) 
The electrochemical potential for the Gold contact is 5.5eV. Energy levels far below the µ are 
always filled so that f0 =1, while energy levels far above µ are always empty with f0 =0. Energy 
levels within a few KBT of µ are occasionally full and occasionally empty so that the average no. of 
electrons lies between 0 and 1: 0 < f0 <1, and these are the levels which participate in conduction 
and current flows. 
Now current flow under bias will depend on the number of energy levels available around E= µ. It 
does not matter whether the energy levels are occupied or empty [42]. And if the empty levels are 
available near E = µ, it will make the molecular device n-type and if filled levels are in close 
proximity, then it will act as p-type. 
 
5.3. A Frontier Molecular Orbital Approach to Electron Transport: In considering the frontier 
molecular orbitals the electron transport through a π-conjugated molecular wire can be explained 
easily. In case of molecular wire in the electronic circuit, when the bias voltage will be applied, one 
electron will be loaded from the cathode to the LUMO of the molecule. According to the Huckel’s 
theory due to the extended π-conjugation present in the molecule, the LUMO will be fully 
delocalized all over the molecule and thus can create the channel for electrical conduction. In other 
words, the loaded electron in the molecule can freely travel from one part to the other with the 
sustained delocalized π-type nature of the LUMO and finally escapes to the other electrode thus 
Solid State Phenomena Vol. 222 111
showing the current in the outer circuit. In a simpler way, the electron transport process in a 
molecular wire can be visualized as if it is channeled from one end of a molecule to the other via π-
type orbitals lying above and below the plane of the molecule. But in case of rectifier, the 
visualization is not as simple as explained in the case of molecular wires. Rectifying behavior of the 
molecular diode can be well explained by considering the effect of forward as well as reverse bias 
conditions on the energy levels of the above molecule. 
 
5.3.1. Forward-bias condition: Due to the chemical perturbation, the donor & acceptor halves of 
the molecule have a difference in the π-orbitals relative energetic positions and this difference in 
energy levels creates the drive for a favorable electron transport. Another driving force for such an 
electron transport (tunneling) through the molecule is the differential spatial location of the frontier 
molecular orbital due to the presence of the σ-bridge. In the forward bias voltage, a higher voltage 
on the donor side and lower voltage on the acceptor side can be realized and hence the donor part 
energies fall and the acceptor energies rise. For electrons to be able to tunnel from the metal contact 
into the empty LUMO of the acceptor, the only thing needed is a sufficient bias voltage to raise the 
Fermi energy of the gold contact on the acceptor side (left electrode by convention) to at least the 
same level as the energy of the LUMO of the acceptor part of the molecule. Once electrons will be 
loaded from the acceptor contact to the acceptor LUMO, tunneling can occur through the semi-
insulating barrier and escape into the right electrode through the donor part. In this condition of 
high voltage on the donor half and a low voltage on the acceptor half, the Fermi energy of the 
acceptor contact aligns with the energy of the LUMO in the acceptor half of the molecule and the 
acceptor LUMO aligns with the donor LUMO. This favorable energy level alignment opens the 
path to the resonant transmission of the electron, and needs a very small bias voltage to raise the 
Fermi energy enough to exceed the LUMO energy of the acceptor half of the molecule in the 
forward bias case. 
 
5.3.2. Reverse-bias condition: In the reverse bias condition, the acceptor part will be having the 
higher voltage and the donor half, lower voltage. Due to this, Fermi energy of the donor side will be 
raised and in the acceptor side, it will fall. At the same time due to the chemical effect of the donor 
group, the increase in energy levels of the donor half will be more pronounced. As a result of this, a 
significantly larger bias voltage will be required to raise the Fermi energy level of the metal contact 
on the donor side to exceed the LUMO energy of the acceptor part of the molecule. 
 
Analysis of the forward as well as reverse bias conditions shows that the applied bias required in the 
reverse bias condition to achieve the transport through a D-B-A type of organic molecule, where the 
bridge acts as a barrier, is much higher compared to the forward bias condition. This is a typical 
diode behavior. 
 
6. Conclusions 
Innovation in semiconductor industry will let present Complementary-Metal-Oxide-Semiconductor 
(CMOS) technology sustain the miniaturization for another decade but finally scaling down will 
require to shift from top down approach to bottom up fabrication techniques. At first, successor 
components will substitute conventional semiconductor analogues only partially on a chip and will 
therefore need to be CMOS –compatible. Moreover, they will have to meet the performance 
specifications dictated by the prevalent technology road map in terms of device size (<10nm), 
performance (>106 operational cycles for memories) and power efficiency (ideally zero power 
dissipation when idle) [6]. However, already the contact resistance of short molecules (such as 
organic molecules) is very high, which means that molecules behave more like insulators than 
conductors, thus rendering low power electronic and energy-harvesting thermoelectric applications 
rather unlikely at present. 
 
112 Nanomaterials
To boost conductance several groups have developed alternative concepts in which a molecule is 
anchored to an electrode through multiple linkers [43, 44] in order to reduce individual bond 
fluctuations and shift the transport-limiting barriers from the fluctuation-prone molecule/metal 
interface onto the more deterministic molecular backbone. 
 
It is also suggestedthat the electrodes could be the main source of fluctuations. An obvious first 
step is to relinquish Au - a material non grata in semiconductor fabrication - and use less diffusive 
metals such as Pt, Pd, Ti or Cr. Well-defined small gaps are, however, more difficult to achieve in 
these materials and involve challenging fabrication methods, such as electromigration [45]. 
 
Carbon-based materials, like nanotubes or graphene, have also shown a better suited option as 
electrode materials for molecular electronics [46]. Graphene in particular is currently considered a 
viable candidate for the post-CMOS era [47] as monolayers can be grown defect-free at the wafer 
scale by chemical vapor deposition. Nanometre-scale gaps have been obtained in graphene by 
atomic force microscopic nanolithography, electron-beam lithography, He ion lithography or 
electro-burning [18]. Pioneering molecular electronics work on graphene has already revealed 
stable molecular orbital gating at room temperature in graphene–molecule–graphene junctions, a 
phenomenon that was disbelieved in the metal–molecule–metal counterpart because of gate 
screening issues by the thick electrodes [18]. Anchor groups that attach molecules to graphene by 
means of direct covalent bonding [48] promises low-ohmic contacts. 
 
Molecular electronics is therefore well placed to help resolve issues related to the scaling down of 
semiconductor-based technologies: namely, increasing device variability and rampant costs for 
materials and fabrication. However, molecular electronics is a long way from practical 
implementation because of the lack of control over the decisive physical and chemical parameters 
of the transport junction. Carbon-based architectures with multiple junctions working together could 
provide a more robust platform for a deterministic assembly of molecules and may help validate 
molecular–electronic concepts and design rules for room-temperature operation. Multidisciplinary 
efforts involving theory, synthetic chemistry, device fabrication and physics, and electrical 
engineering will be required to realize the initial promise of molecular electronics. 
Acknowledgements 
The authors acknowledge the funding support from CSIR in Nanotechnology: Impact on Safety, 
Health and Environment Programme (CSIR-NANOSHE), and academic support from Central 
Scientific Instruments Organization (CSIR-CSIO), Academy of Scientific and Innovative Research 
(AcSIR). Authors also acknowledge SRF- fellowship support from RGNF-UGC, ICMR, and CSIR. 
 
References 
 
[1] N. S. Hush, An overview of the first half-century of molecular electronics, Ann. New York 
Acad. Sci., 1006 (2003) 1-20. 
 
[2] M. Ratner, History of molecular electronics, Nature Nanotechnology, 8 (2013) 378-380. 
 
[3] A. Aviram, M. A. Ratner, Molecular rectifiers, Chemical Physics Letters, 29 (1974) 277-283. 
 
[4] M. D. Stenner, D. J. Gauthier, and M. A. Neifeld, The speed of information in a ‘fast-
light’optical medium, Nature, 425 (2003) 695-698. 
 
[5] S. E. Kubatkin, A. V. Danilov, H. Olin, and T. Cleason, Tunneling through a single quench-
condensed cluster., Journal of Low Temperature Physics, 118 (2000) 307-316. 
 
Solid State Phenomena Vol. 222 113
[6] E. Lörtscher, Wiring molecules into circuits, Nature Nanotechnology, 8 (2013) 381-384. 
 
[7] A. Notargiacomo, V. Foglietti, E. Cianci, G. Capellini, M. Adami, P. Faraci, F. Evangelisti, and 
C. Nicolini, Atomic force microscopy lithography as a nanodevice development technique, 
Nanotechnology, 10 (1999) 458-463. 
 
[8] W. Chen, H. Ahmed, and K. Nakazoto, Coulomb blockade at 77 K in nanoscale metallic islands 
in a lateral nanostructure, Applied Physics Letters, 66 (1995) 3383-3384. 
 
[9] J. Park, A. N. Pasupathy, J.I. Goldsmith, C. Chang, Y. Yaish, J. R. Petta, M. Rinkoski, et al., 
Coulomb blockade and the Kondo effect in single-atom transistors, Nature, 417 (2002) 722-
725. 
 
[10] J. Moreland, and J. W. Ekin, Electron tunneling experiments using Nb‐Sn ‘‘break’’ 
 junctions, Journal of applied physics, 58 (1985) 3888-3895. 
 
[11] G. J. Dolan, Offset masks for lift‐off photoprocessing, Applied Physics Letters, 31 (1977) 337-
339. 
 
[12] D. L. Klein, P. L. Mceuen, J. E. B. Katari, R. Roth and A. P. Alivisatos, An approach to 
electrical studies of single nanocrystals, Applied Physics Letters, 68 (1996) 2574-2576. 
 
[13] Y. Naitoh, T. T. Liang, H. Azehara and W. Mizutani, Measuring molecular conductivities 
using single molecular-sized gap junctions fabricated without using electron beam lithography, 
Japanese journal of applied physics, 44 (2005) L472. 
 
[14] L. F. Sun, S. N. Chin, E. Marx, K. S. Curtis, N. C. Greenham, and C. J. B. Ford, Shadow-
evaporated nanometre-sized gaps and their use in electrical studies of nanocrystals, 
Nanotechnology, 16 (2005) 631-634. 
 
[15] T. Blom, K. Welch, M. Stromme, E. Coronel and K. Leifer, Fabrication and characterization of 
highly reproducible, high resistance nanogaps made by focused ion beam 
milling, Nanotechnology, 18 (2007) 285301-285307. 
 
[16] S. Zhang, S. W. Chung, and C. A. Mirkin, Fabrication of sub-50-nm solid-state nanostructures 
on the basis of dip-pen nanolithography, Nano Letters, 3 (2003) 43-45. 
 
[17] D. Wei, Y. Liu, L. Cao, Y. Wang, H. Zhang, and G. Yu, Real time and in situ control of the 
gap size of nanoelectrodes for molecular devices, Nano letters, 8 (2008) 1625-1630. 
 
[18] F. Prins, A. Barreiro, J. W. Ruitenberg, J. S. Seldenthuis, N. A. Alcalde, L. M. K. Vandersypen 
and H. Zant, Room-temperature gating of molecular junctions using few-layer graphene 
nanogap electrodes, Nano letters, 11 (2011) 4607-4611. 
 
[19] X. Wang, Q. Li, J. Xie, Z. Jin, J. Wang, Y. Li, K. Jiang, and S. Fan, Fabrication of ultralong 
and electrically uniform single-walled carbon nanotubes on clean substrates, Nano letters, 9 
(2009) 3137-3141. 
 
[20] M. C. Hersam, Progress towards monodisperse single-walled carbon nanotubes, Nature 
Nanotechnology, 3 (2008) 387-394. 
 
114 Nanomaterials
[21] N. Izard, S .Kazaoui, K. Hata, T. Okazaki, T. Saito, S. Ijima, and N. Minami, Semiconductor-
enriched single wall carbon nanotube networks applied to field effect transistors., Applied 
Physics Letters, 92 (2008) 243112. 
 
[22] L. Mieux, C. Melburne, M. Roberts, S. Barman, Y. W. Jin and Z. Bao, Self-sorted, aligned 
nanotube networks for thin-film transistors, Science, 321 (2008) 101-104. 
 
[23] G. Kanwar, P. B. Agarwal, and S. Yadav, Comparative study of SWNTs dispersion in organic 
solvent and surfactant along with observation of multilayer Graphene, Physics of 
semiconductor Devices, Springer International Publishing, (2014) 603-606. 
 
[24] G. Zhang, P. Qi, X. Wang, Y. Lu, X. Li, R. Tu, S. Bangsarutip, D. Mann, L. Zhang, and H. 
Die, Selective etching of metallic carbon nanotubes by gas-phase reaction, Science, 314 
(2006) 974-977. 
 
[25] S. Heinze, J. Tersoff, R. Martel, V. Derycke, J. Appenzeller, and P. Avouris, Carbon nanotubes 
as Schottky barrier transistors, Physical Review Letters, 89 (2002) 106801-106804. 
 
[26] A. Javey, J. Guo, Q. Wang, M. Lundstrom, and H. Da, Ballistic carbon nanotube field-effect 
transistors, Nature, 424 (2003) 654-657. 
 
[27] R. T. Weitz, U. Zschieschang, F. Effenberger, H. Klauk, M. Burghard, and K. Kern, High-
performance carbon nanotube field effect transistors with a thin gate dielectric based on a self-
assembled monolayer, Nano letters, 7 (2007) 22-27. 
 
[28] S. Ilani, L. A. K. Donev, M. Kindermann, and P. L. McEuen, Measurement of the quantum 
capacitance of interacting electrons in carbon nanotubes, Nature Physics, 2 (2006) 687-691. 
 
[29] L. Gao, W. Ren, H. Xu, L. Jin, Z Wang, T. Ma, Z. Zhang, Q. Fu, et al., Repeated growth and 
bubbling transfer of graphene with millimetre-size single-crystal grains using platinum, Nature 
communications, 3 (2012) 699-706. 
 
[30] A. H. C. Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, and A.K. Geim, The electronic 
propertiesof graphene, Rev. Mod. Phys., 81 (2009) 109-162. 
 
[31] M. C. Lemme, T. J. Echtermeyer, M. Baus, and H. Kurz, A Graphene field effect device, 
IEEE Elec. Dev. Lett., 28 (2007) 1-12. 
 
[32] S. M. M. Dubois, Z. Zanolli, X. Declerck, and J. C. Charlier, Electronic properties and 
quantum transport in Graphene-based nanostructures, Eur. Phys. J. B, 72 (2009) 1–24. 
 
[33] F. O. Yang, B. Huang, Z. Li, J. Xiao, H. Wang, and H. Xu, Chemical functionalization of 
graphene nano ribbons by carboxyl groups on Stone Wales Defects, J. Phys. Chem. C, 112 
(2008) 12003–12007. 
 
[34] G. P. Tang, J. C. Zhou, Z. H. Zhang, X. Q. Deng, and Z. Q. Fan, Altering regularities of 
electronic transport properties in twisted graphene nanoribbons, Applied Physics Letters, 101 
(2012) 023104 1-5. 
 
[35] A. Yipeng, and Z. Yang, Abnormal electronic transport and negative differential resistance of 
graphene nanoribbons with defects, Appl Phys Lett, 99 (2011) 192102 1-3. 
 
Solid State Phenomena Vol. 222 115
[36] K. Yousuke, K. Fukui, T. Enoki, and K. Kusakabe, Edge state on hydrogen-terminated graphite 
edges investigated by scanning tunneling microscopy, Phys Rev B, 73 (2006) 125415-125424. 
 
[37] L. Zuanyi, Q. Haiyun, W. Jian, L. G. Bing, and W. Duan, Role of Symmetry in the Transport 
Properties of Graphene Nanoribbons under Bias, Phys Rev Lett, 100 (2008) 206802 1-4. 
 
[38] K. Tang, R. Qin, J. Zhou, H. Qu, J. Zheng, R. Fei, H. Li, Q. Zheng, Z. Gao, J. Lu, Electric-
Field-Induced Energy Gap in Few-Layer Graphene, J. Phys. Chem. C, 115 (2011) 9458 –9464. 
 
[39] H. N. Zhen, T. Yu, Y. H. Lu, Y. Y. Wang, Y. P. Feng, and Z. X. Shen, Uniaxial strain on 
graphene: Raman spectroscopy study and bandgap opening, ACS Nano, 2 (2008) 2301–2305. 
 
[40] D. V. Kosynkin, A. L. Higginbotham, A. Sinitskii, J. R. Lomeda1, A. Dimiev, B. K. Price, and 
J. M. Tour, Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons, Nature, 
4 (2010) 5405–5413. 
 
[41] X. Wang, Y. Ouyang, X. Li, H. Wang, J. Guo, and H. Dai, Room-Temperature All-
Semiconducting Sub-10-nm Graphene Nanoribbon Field-Effect Transistors, Phys Rev Lett, 100 
(2008) 206803 1-4. 
 
[42] S. Datta, Quantum transport: atom to transistor, Cambridge University Press, 2005. 
 
[43] F. Worchem, D. Gao, F. Scholz, H. Nothofer, G. Nelles, and J. Wessels, Efficient electronic 
coupling and improved stability with dithiocarbamate-based molecular junctions., Nature 
nanotechnology, 5 (2010) 618-624. 
 
[44] C. Martin, D. Ding, J. Sorensen, T. Bjornholm, J. Ruitenbeek, and H. Zant, Fullerene-based 
anchoring groups for molecular electronics, Journal of the American Chemical Society, 130 
(2008) 13198-13199. 
 
[45] H. Park, A. Lim, A. Paul, J. Park and P. Muceun, Fabrication of metallic electrodes with 
nanometer separation by electromigration, Applied Physics Letters, 75 (1999) 301-303. 
 
[46] C. Marquardt, S. Gurender, A. Baszczyk, S. Dehm, F. Henrich, H. Lohneysen, M Mayor, and 
R. Krupke, Electroluminescence from a single nanotube-molecule-nanotube junction, Nature 
nanotechnology, 5 (2010) 863-867. 
 
[47] International Roadmap Committee, International technology roadmap for semi-
conductors, www.itrs.net/Links/2003ITRS/ExecSum2003.pdf (2003). 
 
[48] Y. Cao, S. Dong, S. Liu, P. Z. Liu, and P. X. Guo, Toward functional molecular devices based 
on Graphene–molecule junctions, Angewandte Chemie International Edition, 52 (2013) 3998-
4002. 
 
 
 
116 Nanomaterials
 
 
General rights 
Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright 
owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. 
 
 Users may download and print one copy of any publication from the public portal for the purpose of private study or research. 
 You may not further distribute the material or use it for any profit-making activity or commercial gain 
 You may freely distribute the URL identifying the publication in the public portal 
 
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately 
and investigate your claim. 
 
 
 
 
 
Downloaded from orbit.dtu.dk on: set. 28, 2023
Multi Scale Micro and Nano Metrology for Advanced Precision Moulding Technologies
Quagliotti, Danilo
Publication date:
2017
Document Version
Publisher's PDF, also known as Version of record
Link back to DTU Orbit
Citation (APA):
Quagliotti, D. (2017). Multi Scale Micro and Nano Metrology for Advanced Precision Moulding Technologies.
Technical University of Denmark.
https://orbit.dtu.dk/en/publications/8137549a-e119-4b43-87d2-3819b03a74b2
Multi Scale Micro and Nano Metrology for 
Advanced Precision Moulding Technologies 
Danilo Quagliotti 
 
PhD Thesis 
Kongens Lyngby, February 2017 
 
 
 
Technical University of Denmark 
Department of Mechanical Engineering – 
Manufacturing Engineering 
Produktionstorvet, building 427S, 
2800 Kongens Lyngby, Denmark 
www.mek.dtu.dk 
Phone: (+45) 4525 4763 
Fax: (+45) 4593 0190 
E-mail: info@mek.dtu.dk 
Published in Denmark by 
Technical University of Denmark 
Copyright © D. Quagliotti 2017 
All rights reserved 
 
ISBN: 978-87-7475-480-0 
 
http://www.mek.dtu.dk/
mailto:info@mek.dtu.dk
 
Ecce et naves, cum tam magnae sint 
et a ventis validis minentur, 
circumferentur a minimo gubernaculo, 
ubi impetus dirigentis voluerit. 
Epistula Jacobi 3,4. 
 
 
Preface 
This thesis was prepared as one of the requirements of the Ph.D. course of study at the Technical 
University of Denmark (DTU), Department of Mechanical Engineering. The work was carried 
out from February 2014 to October 2016 and from January to February 2017 at the Department 
of Mechanical Engineering, at the Technical University of Denmark (DTU), under the supervision 
of Associate Prof. Guido Tosello and Prof. Hans Nørgaard Hansen. 
From October to December 2016, nine weeks were spent at The University of Nottingham (UK), 
Faculty of Engineering, under the supervision of Prof. Richard Leach. 
 
I would like to thank all my supervisors for their inspirational contribution to my work. In 
particular, I would like to express my gratitude to Associate Prof. Guido Tosello for his constant 
support and motivation, his highly valuable contribution to my work and the endless opportunity 
I was given during the three years of his great supervision. 
My gratitude also goes to Prof. Hans Nørgaard Hansen for his inspiration and his wise and 
constant guidance. 
I would like to thank Prof. Richard Leach for the great hospitality at The University of Nottingham 
and for giving me the opportunity to collaborate with the Manufacturing Metrology Team. 
 
The work was funded by the Technical University of Denmark. Financial support received by the 
“Hi-Micro” European project, 7th Framework Programme (FP7‐2012‐NMP‐ICT‐FoF: 314055—
www.hi-micro.eu), by the Danish foundation Fabriksejer, Civilingeniør Louis Dreyer Myhrwold 
og hustru Janne Myhrwolds Fond, by the Danish foundation Otto Mønsteds Fond and by the 
Danish foundation Idella Fond is gratefully acknowledged. 
 
 
 
Kgs. Lyngby, February 2017 
 
Danilo Quagliotti 
 
 
i 
http://www.hi-micro.eu/
ii PREFACE 
 
 
Abstract 
The technological revolution that has deeply influenced the manufacturing industry over the past 
two decades opened up new possibilities for the realisation of advanced micro and nano systems 
but, at the same time, traditional techniques for quality assurance became not adequate any longer, 
as the technology progressed. 
The gap between the needs of the manufacturing industry and the well-organized structure of the 
dimensional and geometrical metrology appeared, above all, related to the methodologies and, 
also, to the instrumentation used to deal with the incessant scaling down of the critical dimensions 
of the novel microand nano production. 
Nowadays, design methodologies and concurrent tolerance guidelines are not yet available for 
advanced micro manufacture. Moreover, there are no shared methodologies that deals with the 
uncertainty evaluation of feature of size in the sub-millimetre scale. 
On the other hand, a large choice of measurement equipment is now available but limitations in 
their use and of the instruments themselves are, in many cases, not completely understood, yet. 
In this context, the ambition of the PhD project was to develop and implement a complete 
metrological framework for advanced precision micro moulded products with micro/nano 
structured surfaces and micro/nano geometries, across several length scales. 
Uncertainty evaluation and traceability, specification intervals formulation, assessment of the 
moulded parts replication and a deep investigation on the optical instruments currently available 
for micro/nano dimensional and geometrical measurements were all subjects of the research 
conducted during the three years of the PhD course of study and that were collected in this final 
work. 
Traceability and uncertainty evaluation were dealt with the development of a comprehensive 
statistical methodology based on the well-known frequentist approach. It was successfully applied 
to dimensional and geometrical measurements in the micro/nano length scale. 
A novel method was developed on purpose for the formulation of specification intervals. Based 
on the evaluation of the shrinkage uncertainty, it allows to discriminate between the shrinkage of 
1D and 2D features and cope with the influence of length scale. The method was applied and 
validated in the specific case of a micro-powder injection moulding production. Nevertheless, it 
is of general validity for any moulding process in which the material undergoes a change in 
dimensions from the mould cavity, due to a phase transformation. In parallel to the formulation 
of specification intervals, an investigation of two instruments with two different working principle 
proved a mutual dependence between the quality of the measurement process and the quality of 
the production. The measurement process influenced the quality assurance, but the lack of quality 
of the parts influenced the measurement process. 
The surface texture replication was investigated about the amplitude (Sa, Sq) and the slope (Sdq) 
and assessed by the replication fidelity, i.e., comparing the produced parts with the tool used to 
replicate the geometry and evaluating the measurement uncertainty. The evaluation included the 
repeatability and reproducibility of the production process, the amplitude and slope replication of 
the features on the surface, the evaluation of the uncertainty of the replication fidelity. 
The investigation of optical instruments started with the processing of the data of an international 
comparison of surface texture measurements, in the sub-micrometre scale, by optical instruments, 
organised under the umbrella of the Scientific Technical Committee on ‘Surfaces’ (STC-S) of 
The International Academy for Production Engineering (CIRP). The comparison unveiled the 
state-of-the-art performance, in the sub-micrometre scale, of the three main microscopes working 
principle currently used in areal topography measurement (confocal microscopy, coherent 
scanning interferometry and focus variation microscopy). Results showed that agreement between 
iii 
iv ABSTRACT 
optical instruments and reference measurements (by atomic force microscopy) could be reached 
to some extent, largely depending on the technology of the instruments used. 
The limitations of the performance of the optical instruments were, also, inspected in specific 
cases that can arise during practical operation and that are becoming more and more common in 
modern micro and nano manufacturing. Several environmental sources were identified (thermal 
drifts, air conditioning system, stray light), which can introduce substantial environmental noise 
into the measurements, but, also, internal noise related to a prolonged use of an instrument. 
 
 
 
 
 
 
Acknowledgements 
During the course of my Ph.D. studies several experts were involved in various ways in my 
research. I wish to express them my gratitude for their commitment to the project. 
I would like to thank 
• Dr. Tech. Prof. Leonardo De Chiffre at Department of Mechanical Engineering of the 
Technical University of Denmark for encouraging and introducing me into this field. 
• Mr. René Sobiecki and Mr. Jakob Rasmussen at Department of Mechanical Engineering 
of the Technical University of Denmark for their unique support and involvement in many 
of my activities. 
• Prof. Giulio Barbato and Dr. Gianfranco Genta at Politecnico di Torino for fruitful 
discussions and for sharing with me their knowledge in statistical analysis and uncertainty 
evaluation. 
• Dr.-Ing. Henning Zeidler and Dipl.-Ing. André Martin at the Technical University of 
Chemnitz for their collaboration in relation to a large number of time-consuming 
measurements. 
• Dr.-Ing. Oltmann Riemer and Dr. Carla Flosky at the University of Bremen for their 
collaboration in relation to a large number of time-consuming measurements. 
• Dr. Jun Qian and Dr. Marius Nabuurs at the University of Leuven in relation to a large 
number of time-consuming measurements. 
• Dr. Stefania Gasparin at The LEGO Group for fruitful discussions and collaboration in 
many activities. 
• Dr. Jørgen Garnæs and Dr. Poul Erik Hansen at the Danish Metrology Institute DFM for 
their valuable collaboration with atomic force microscopy and confocal microscopy. 
• Dr. Wahyudin P Syam and Dr. Xiaobing Feng, Manufacturing Metrology Team at The 
University of Nottigham, for the great time spent together and fruitful discussions. 
• Dr. François Blateyron at Digital Surf for his valuable and unique support with 
MountainsMap® software. 
• Dr. Jan Friis Jorgensen and Dr. Anders Kühle at Image Metrology for their valuable and 
unique support with SPIP™ and topoStitch™ software. 
 
Last, but not the least, I wish to thank the M.Sc. and B.Sc. project students I supervised during 
my Ph.D. for their commitment to the project, their results and their friendship: 
Mr. Federico Baruffi, Mr. Jacek Salaga, Mr. Michael Gani, Mr. Martin Kain and Mr. Dario 
Loaldi. Their thesis works represented an important contribution to the project: 
 
• Baruffi F 2016 Optical metrology and precision milled metal surfaces for process 
validation and product quality assurance, M.Sc. Thesis, DTU. 
• Salaga J 2016 Dimensional and surface micro/nano metrology for precision moulding 
technology, M.Sc. Thesis, DTU. 
• Gani M and Kain M 2016 Design and Optimization of High Precision Positioning Fixture, 
B.Sc. Thesis, DTU. 
• Loaldi D 2017 Technological signature in precision injection compression moulding of 
polymer Fresnel lenses, M.Sc. Thesis, DTU. 
 
 
v 
vi ACKNOWLEDGEMENTS 
 
 
 
 
Table of content 
TABLE OF CONTENTS 
 
1. INTRODUCTION 
1.1 BACKGROUND .......................................................................................................................... 1 
1.1.1 PRECISION AND MICRO INJECTION MOULDING PROCESS TECHNOLOGIES ...................................................... 1 
1.2 MOTIVATION ............................................................................................................................ 2 
1.2.1 PROBLEM IDENTIFICATION ................................................................................................................. 3 
1.2.2 PROJECT OBJECTIVES AND SCIENTIFIC METHODS ..................................................................................... 3 
1.2.3 COLLABORATIONS ............................................................................................................................ 3 
1.3 STRUCTURE OF THE WORK ........................................................................................................4 
REFERENCES .......................................................................................................................................... 5 
 
2. METROLOGY FOR ADVANCED PRECISION MOULDING TECHNOLOGIES 
2.1 INTRODUCTION ......................................................................................................................... 7 
2.2 INSTRUMENTATION FOR MICRO AND NANO METROLOGY ....................................................... 7 
2.2.1 CONTACT INSTRUMENTS ................................................................................................................... 7 
2.2.2 SCANNING PROBE AND PARTICLE BEAM MICROSCOPY .............................................................................. 8 
2.2.3 INTERFEROMETRY .......................................................................................................................... 10 
2.2.4 OPTICAL INSTRUMENTS................................................................................................................... 12 
2.2.4.1 Optical scanning instruments .......................................................................................... 12 
2.2.4.2 Areal optical instruments ................................................................................................. 15 
2.2.5 COORDINATE METROLOGY............................................................................................................... 18 
2.2.6 COMPUTER TOMOGRAPHY .............................................................................................................. 20 
2.3 CALIBRATION, TRACEABILITY AND UNCERTAINTY EVALUATION ............................................. 21 
2.3.1 UNCERTAINTY EVALUATION ............................................................................................................. 22 
2.4 TOLERANCE INTERVALS SPECIFICATION .................................................................................. 25 
2.5 SURFACE TEXTURE PARAMETERS ............................................................................................ 26 
REFERENCES ........................................................................................................................................ 27 
 
3. ADVANCED PRECISION INJECTION MOULDING TECHNOLOGIES 
3.1 INTRODUCTION ....................................................................................................................... 31 
3.2 INJECTION MOLDING .............................................................................................................. 33 
3.2.1 MICRO INJECTION MOULDING .......................................................................................................... 33 
3.3 POWDER INJECTION MOULDING............................................................................................. 34 
3.4 COMPRESSION INJECTION MOULDING ................................................................................... 35 
REFERENCES ........................................................................................................................................ 36 
 
vii 
viii TABLE OF CONTENT 
4. UNCERTAINTY ASSESSMENT FOR MICRO NANO DIMENSIONAL AND TOPOGRAPHIC 
MEASUREMENTS 
4.1 INTRODUCTION ...................................................................................................................... 39 
4.2 METROLOGY OF MICRO TOOL INSERT .................................................................................... 40 
4.2.1 MEASUREMENTS PROCESSING .......................................................................................................... 42 
4.2.2 STATISTICAL ANALYSIS ..................................................................................................................... 45 
4.2.2.1 Outliers detection ............................................................................................................. 46 
4.2.2.2 Systematic effects ............................................................................................................ 49 
4.2.3 UNCERTAINTY EVALUATION ............................................................................................................. 56 
4.3 CORRECTION OF THE SYSTEMATIC BEHAVIOUR VS. TIME SEQUENCE ..................................... 60 
4.4 CORRECTION OF SYSTEMATIC BEHAVIOUR IN TOPOGRAPHICAL SURFACE ANALYSIS ............. 65 
4.4.1 METROLOGY ................................................................................................................................. 65 
4.4.2 RESULTS ...................................................................................................................................... 68 
4.5 DISCUSSION ............................................................................................................................ 75 
4.6 CONCLUSION .......................................................................................................................... 76 
4.7 OUTLOOK ............................................................................................................................... 76 
REFERENCES ........................................................................................................................................ 77 
 
5. SHRINKAGE CALIBRATION METHOD FOR FORMULATION OF MULTISCALE DIMENSIONAL 
TOLERANCE SPECIFICATIONS 
5.1 INTRODUCTION ...................................................................................................................... 79 
5.2 CERAMIC INJECTION MOULDING MANUFACTURED PARTS ..................................................... 80 
5.3 METROLOGY FOR QUALITY ASSURANCE ................................................................................. 81 
5.4 FORMULATION OF SPECIFICATION INTERVALS ....................................................................... 89 
5.5 ASSESSMENT OF MULTISCALE MEASUREMENTS BY TWO WORKING PRINCIPLES ................... 92 
5.6 DISCUSSION ............................................................................................................................ 96 
5.7 CONCLUSION .......................................................................................................................... 97 
5.8 OUTLOOK ............................................................................................................................... 98 
REFERENCES ........................................................................................................................................ 98 
 
6. REPLICATION ASSESSMENT OF POLYMER SURFACES AT SUB-MICROMETRE SCALE 
6.1 INTRODUCTION .................................................................................................................... 101 
6.2 AMPLITUDE REPLICATION OF NANOSTRUCTURED POLYMER SURFACES ............................... 101 
6.2.1 MANUFACTURE OF THE SUB-MICRO STRUCTURED POLYMER SURFACES ................................................... 102 
6.2.2 REPEATABILITY OF THE REPLICATION PROCESS .................................................................................... 104 
6.2.3 REPRODUCIBILITY OF THE REPLICATION PROCESS ................................................................................ 105 
6.2.4 UNCERTAINTY MODEL ................................................................................................................... 105 
6.2.5 REPLICATION FIDELITY ................................................................................................................... 107 
6.3 AMPLITUDE AND SLOPE REPLICATION OF SUB-MICRO POLISHED POLYMER SURFACES ........ 111 
6.3.1 METROLOGY TASK AND UNCERTAINTY EVALUATION ............................................................................ 111 
6.3.2 REPLICATION FIDELITY ................................................................................................................... 114 
6.4 DISCUSSION ..........................................................................................................................