Nanotechnology and nanomaterials represents a multi-billion industry today and will continue to grow over the next few years, driven by demand from the electronics, energy, chemicals, life coatings and catalysts sectors. Markets nanotechnology is leading to growth opportunities in are covered in this report including:

1. INTRODUCTION 

The advances in ultra-large-scale integration (ULSI) technology mainly have been based on downscaling of the minimum feature size of complementary metal-oxide semiconductor (CMOS) transistors. The limit of scaling is approaching and there are unsolved problems such as the number of electrons in the device’s active region. If this number is reduced to less than 10 electrons (or holes), quantum fluctuation errors will occur and the gate insulator thickness will become too small to block quantum mechanical tunneling, which may result in unacceptably large leakage currents. On the other hand, the recent evolution of nanotechnology may provide opportunities for novel devices, such as single-electron devices, carbon nanotubes, Si nanowires, and new materials, which may solve these problems. Utilization of quantum effects and ballistic transport characteristics also may provide novel func- tions for silicon-based devices. Among various candidate materials for nanometer scale devices, silicon nanodevices are particularly promising because of the existing silicon process infrastructure in semiconductor industries, the compatibility to CMOS circuits, and a nearly perfect interface between the natural oxide and silicon. 

Nanomaterials for energy production and storage, nanoparticles for drug delivery and biosensors for diagnostics… Nanotechnology is an emerging technology with applications in almost all sectors, and it is expected to lead the future of technological development. However, as a relatively new field, the use of research results is still in its early stages and there were no good practices identified, especially from a multidisciplinary perspective (research centers, venture capitalists and experts in intellectual protection).

Models of traditional technology transfer were probably not applicable for the “nano” area. Therefore, it was considered fundamental to carry out a research project in this subject because transfer models are essential for any sector related to R&D in order to market and protect the research results appropriately.

As Intellectual Property Rights can be applied to different stages of the product development, from basic research to commercialisation, the first step was to examine the value chains and the appropriate IP model for the technology transfer for twelve selected case studies from five industries: electronics, energy, life sciences, materials, and water and environment. The Nano2Market partners then conducted a Technology Mapping exercise in order to list the areas of the “nano” innovations. Models and strategies were validated by a panel of experts.

As a result, the project has provided an Intellectual Property Guide for nanotechnology research, which develops guidelines for licensing agreements and consortium and comments and suggestions to manage IP in nanotechnology projects. The guidelines depend on the type of application, level of development of the research, and the size of the organisation (small or large research institutions, or small, medium or large enterprises). The size is important because partners found that smaller organisations employed different strategies because the funds were not available. People are generally more confident working, investing and collaborating with larger institutions or enterprises but the smaller ones must wait until the technology is in a more advanced stage before licensing.

Nano2market has also designed an interactive Toolbox, which provides tools for companies that wish to collaborate on nanotech projects and tools for investors (business angels, venture capitalists and corporate venture capitalists). This instrument offers best practices reports for businesses and useful information on how to place a development in the market thanks to an adequate business plan for entrepreneurs.

Coordinated from the Office of International Projects at the University of Alicante (Spain), the project Nano2Market (Best Practices for IPR and Technology Transfer in Nanotechnology development) lasted a year and had a budget of 700.000 euros, financed by the European Commission under the Seventh Framework Programme.

Characterizing devices at low current levels requires knowledge, skill, and the right test equipment. Even with all three, achieving accuracy in these measurements can be a challenge because the current level is often at or below the noise level of the test setup. To ensure measurement accuracy, it is important to know the type of test equipment to use, the different sources of measurement error, and the appropriate techniques to minimize these errors. Examining several test examples, such as characterization of a field-effect transistor (FET) and a carbon nanotube, can help in the learning process.

The term low current is relative, of course. A current level considered low for one application, such as 1mA, may be high for a device operating at 10nA. In general, an instrument’s noise level will establish its low-level sensitivity, with low current measurements referring to those made near an instrument’s noise level. Trends in portable and remote electronic devices, along with advances in semiconductors and nanotechnology, are requiring greater use of low current measurements. Small geometry devices, photovoltaic devices, and carbon nanotubes (CNTs) are a few examples of devices designed to operate at extremely low current levels, and all of these devices must be characterized in terms of their current-voltage characteristics (I-V measurements).

A number of instruments are available for low-current measurements, depending on the type of device under test (DUT) and the level of current to be measured. Perhaps the most ubiquitous tool on production lines and in field service is the digital multimeter (DMM), which typically provides capabilities for measuring current, voltage, resistance, and temperature. The range of commercial products is wide, from low-cost units with 3½-digit readout resolution to rack-mount and benchtop high precision laboratory units. The most sensitive DMMs available can measure current levels as low as about 10pA.

When greater precision is needed, various forms of ammeters are available to measure current. These can be as simple as older types that measure current flow from the mechanical deflection of a coil in a magnetic field. More modern digital ammeters use an analog-to-digital converter (ADC) to measure the voltage across a shunt resistor and then determine and display the current from that reading. Newer picoammeters typically use a feedback resistor, which allows more accuracy in current measurements at such low levels. They are available in various configurations, including high-speed models and logarithmic units capable of measuring a wide range of currents. While they are extremely versatile, it is useful to understand the performance limitations of feedback ammeters.

Feedback Ammeter Performance. A simple feedback ammeter can be modeled with a small number of parameters. The current source is modeled as a voltage source in series with a parallel RC circuit, i.e., the source resistance (RS) and parallel source capacitance (CS). The feedback ammeter is modeled as a feedback amplifier with a parallel RC feedback circuit across it (i.e., RF and CF), with the two amplifier inputs being the external current source and the internal voltage noise source, VNOISE. The capacitances in the source and measurement circuits are parasitic elements associated with the resistances and circuit wiring. Using this model and ignoring capacitance, the noise gain of the ammeter circuit can be found from:

Output Voltage Noise = (Input Voltage Noise) x (1 + RF/RS)

As this equation implies, the output of a feedback ammeter circuit is a voltage, which is proportional to the input current. As the source resistance decreases in value, the output noise increases. When RF = RS, the input noise is multiplied by a factor of 2. If the source resistance is too low, it can have a detrimental effect on the noise performance of the measurement system. The optimum source resistance is a function of required measurement range for an ammeter, with a minimum value of 1MegOhm to measure nanoamps of current, compared to a minimum value of 1GigOhm to measure picoamps of current.

However, source capacitance can also affect the noise performance of a low current measurement instrument. In general, as the source capacitance increases, the noise gain also increases. This means that the equation above should be modified by substituting the feedback impedance (ZF) for the feedback resistance (RF) and the source impedance (ZS) for the source resistance (RS).

Additional current measurement instruments include electrometers and source-measure units (SMUs). An electrometer is essentially a voltmeter with a high input impedance (1TOhm and higher) that can be used to measure low current levels. It can be used as an ammeter to measure low current levels even at low voltages, and can also be used as a voltmeter to make voltage measurements with minimal effect on the circuit being measured. As an ammeter, an electrometer can measure currents as low as the instrument’s input offset current, as low as 1fA in some cases. As a voltmeter, an electrometer can measure the voltage on a capacitor without significantly discharging the device, and can measure the potential of piezoelectric crystals and high-impedance pH electrodes.

The SMU is an innovation for making low-current measurements. It combines precision current sources and voltage sources with sensitive detection circuitry for measuring both current and voltage. An SMU can simultaneously provide a source of current and measure voltage or provide a source of voltage and measure current. A well-equipped SMU may include a voltage source, current source, ammeter, voltmeter, and ohmmeter and is also programmable for use in automatic-test-equipment (ATE) systems.

Minimizing External Noise. All of these measuring instruments are effective tools for measuring current, but their sensitivity to low levels of current will be limited mainly by sources of noise, both within and external to the instrument. The DUT also affects the level of current that can be accurately measured with a given instrument, because the DUT’s source resistance (RS) establishes the level of Johnson current noise (IJ), which is low-level noise caused by temperature effects on electrons in a conductor. Johnson noise, which can be expressed in terms of either current or voltage, is essentially the voltage noise of a device divided by the device resistance:

IJ= √(4kTB/RS) / RS,

where

k = Boltzmann’s constant (1.38 × 10–23 J/K),

T = Absolute temperature of the source (in ºK),

B = the noise bandwidth (in Hz), and

RS = the resistance of the source (in ohms).

Both temperature and noise bandwidth affect the Johnson current noise. A reduction in either parameter will also reduce the Johnson current noise. Cryogenic cooling, for example, is often used to reduce noise in amplifiers and other circuits but adds cost and complexity. The noise bandwidth can be reduced by filtering, but this will result in slowing the measurement speed. The Johnson current noise also decreases as the DUT’s source resistance decreases, but this is not often a practical or even possible option.

Ideally, a current measurement would be just that of the DUT source. However, current noise from various unwanted sources can make it difficult to read a low-level DUT source current. One of these unwanted sources is part of the measurement system itself, i.e., the coaxial cables used to interconnect test instruments to each other or to the DUT. Typical test cables can generate as much as tens of nanoamps of current as a result of the triboelectric effect. This occurs when the outer shield of a coaxial test cable rubs against the cable’s insulation when the cable is flexed. As a result, electrons are stripped from the insulation, and added to the current total. In some applications, such as nanotechnology and semiconductor research, the current generated by this effect may exceed the level of current to be measured from the DUT.

Triboelectric effects can be minimized by using low-noise cable, with an inner insulator of polyethylene coated with graphite underneath the outer shield. The graphite reduces friction, and provides a path for the displaced electrons to return to their original locations, eliminating random electron motion and their contribution to the additional noise level. Excess current flow from triboelectric effects can also be minimized by reducing the length of the test cables as much as possible. The test setup should be isolated from vibration to minimize unwanted movement of the test cables, by positioning test cables on top of vibration-absorbing material, such as foam rubber. Test cable movement can also be minimized by taping the cables to a stable surface, such as the test bench.

Piezoelectric effect is another source of error in low-current measurements. It causes spurious current generation due to mechanical stress on susceptible materials. The effect varies by material, although some materials commonly used in electronic systems, such as polytetrafluoroethylene (PTFE) dielectrics, can produce a relatively large amount of current for a given amount of stress and vibration. Ceramic materials are less affected by piezoelectric effects and produce lower current levels. To minimize current generated by this effect, it is critical to minimize mechanical stress on insulators and construct the low-current test system using insulating materials with minimal piezoelectric properties.

Insulators can also degrade low-current measurement accuracy by means of dielectric absorption. This phenomenon occurs when a high-enough voltage across an insulator causes positive and negative charges to polarize. When the voltage is removed from the insulator, it gives up the separated charges as a decaying current, which is added to the total amount measured during a test. The decay time for the current from dielectric absorption to dissipate can be from minutes to hours. The effect can be minimized by applying only low-voltage levels to insulators used for low-current measurements.

Insulators can also degrade low-current measurement accuracy due to contamination from salt, moisture, oil, or even fingerprints on the surface of the insulator. Contamination effects can also plague printed circuit boards in a test fixture or in the test setup when, for example, excessive flux is used when soldering. On an insulator, the contamination acts to form a low-current battery at a sensitive current node within the insulator, generating noise currents that can be on the order of nanoamps. To minimize measurement errors from insulator contamination, an operator should wear gloves when handling insulators or simply avoid touching them. The use of solder should be minimized, and solder areas should be cleaned with an appropriate solvent, such as isopropyl alcohol. A clean cotton swab should be used for every cleaning, and cotton swabs should never be reused or dipped into the cleaning solution after having been used for cleaning.

It is critical to make low-current measurements in the absence of magnetic fields, because such fields can induce current flow in conductors. This is typically due to variations in magnetic field intensity, or motion of a conductor within a magnetic field. Both cases should be avoided to maintain measurement accuracy, which is best accomplished by properly shielding the measuring instrument or system.

Minimizing Instrument Offset Current. An instrument used for low-current measurements should show a zero reading when its input terminals are left in an open-circuit condition. Unfortunately, this is rarely the case due to a small current known as the input offset current. It is caused by bias currents of active devices in measuring instrument circuitry, as well as leakage current through insulators in the instrument or test system. Most instrument manufacturers specify the input offset current on their products’ data sheets for comparison purposes, and this small amount of current must be taken into account in any low-current measurement. In other words, the instrument’s reading is actually the sum of the DUT source current and the instrument’s input offset current.

The input offset current can be found by capping the input connector and selecting the lowest current range available on the measuring instrument. The reading shown by the instrument, after it has properly settled to a stable value, should be within the specification shown on the instrument’s data sheet and can be subtracted from DUT readings. On some instruments, a current-suppression function can partially null input offset current.

Another way to subtract input offset current from a low-current measurement is to use a relative function found on some measuring equipment, such as ammeters. The relative function stores the reading of whatever residual offset current is being measured with the input terminals left in an open-circuit condition; this reading is treated as the zero point for subsequent readings.

Application Examples. Some examples of practical low-current measurements include characterization of field effect transistors (FETs) and CNT devices. A more common FET test involves evaluation of a device’s common-source characteristics. Even at low current levels, the drain current can be studied using a simple test setup with a two-channel SMU, such as the Keithley Series 2600A System SourceMeter instrument. A two-channel SMU has the capability to source current or voltage and measure current or voltage simultaneously. To characterize a FET, it is mounted in a test fixture that allows secure ground and bias connections. One SMU channel supplies a swept gate-source voltage (VGS) to the FET while the other supplies a swept drain-source voltage (VDS) and measures the FET’s drain current (ID). This simple test setup allows the measurement of drain currents as low as 10nA or less.

Electronic materials such as photovoltaic wafers and CNT sheets are typically characterized in terms of their current density—the amount of current they can generate for a given area of material. Researchers from South Korea’s Seoul National University, conduct such tests to evaluate multi-walled carbon nanotube (MWNT) devices fabricated on an arc-discharge CNT substrate using a Keithley Model 6517 electrometer. In these studies, current densities as low as 10–4/cm2 were measured at applied electric fields of 5V/μm and less. Practical analysis of the I-V characteristics of CNT-based electronics can also be performed in a manner similar to that for the FET by using a pair of SMUs to sweep drain and gate voltages while measuring and plotting the drain current as a function of gate voltage.

The required resolution and accuracy of low-current measurements will dictate the type of measurement tool used. When accuracy is less of an issue, a basic DMM may suffice. But for more demanding requirements, a precision electrometer or SMU may be needed. These precision instruments are optimized for low-current measurements, providing measurement resolution as small as 1fA. More techniques and tips on low current measurements are contained in Keithley’s Low Level Measurements Handbook.

As far back as the 1990s, long before anyone had actually isolated graphene – a honeycomb lattice of carbon just one atom thick – theorists were predicting extraordinary properties at the edges of graphene nanoribbons. Now physicists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), and their colleagues at the University of California at Berkeley, Stanford University, and other institutions, have made the first precise measurements of the “edge states” of well-ordered nanoribbons.

A graphene nanoribbon is a strip of graphene that may be only a few nanometers wide (a nanometer is a billionth of a meter). Theorists have envisioned that nanoribbons, depending on their width and the angle at which they are cut, would have unique electronic, magnetic, and optical features, including band gaps like those in semiconductors, which sheet graphene doesn’t have.

“Until now no one has been able to test theoretical predictions regarding nanoribbon edge-states, because no one could figure out how to see the atomic-scale structure at the edge of a well-ordered graphene nanoribbon and how, at the same time, to measure its electronic properties within nanometers of the edge,” says Michael Crommie of Berkeley Lab’s Materials Sciences Division (MSD) and UC Berkeley’s Physics Division, who led the research. “We were able to achieve this by studying specially made nanoribbons with a scanning tunneling microscope.”

The team’s research not only confirms theoretical predictions but opens the prospect of building quick-acting, energy-efficient nanoscale devices from graphene-nanoribbon switches, spin-valves, and detectors, based on either electron charge or electron spin. Farther down the road, graphene nanoribbon edge states open the possibility of devices with tunable giant magnetoresistance and other magnetic and optical effects.

The well-tempered nanoribbon

“Making flakes and sheets of graphene has become commonplace,” Crommie says, “but until now, nanoribbons produced by different techniques have exhibited, at best, a high degree of inhomogeneity” – typically resulting in disordered ribbon structures with only short stretches of straight edges appearing at random. The essential first step in detecting nanoribbon edge states is access to uniform nanoribbons with straight edges, well-ordered on the atomic scale.

Hongjie Dai of Stanford University’s Department of Chemistry and Laboratory for Advanced Materials, a member of the research team, solved this problem with a novel method of “unzipping” carbon nanotubes chemically. Graphene rolled into a cylinder makes a nanotube, and when nanotubes are unzipped in this way the slice runs straight down the length of the tube, leaving well-ordered, straight edges.

Graphene can be wrapped at almost any angle to make a nanotube. The way the nanotube is wrapped determines the pitch, or “chiral vector,” of the nanoribbon edge when the tube is unzipped. A cut straight along the outer atoms of a row of hexagons produces a zigzag edge. A cut made at a 30-degree angle from a zigzag edge goes through the middle of the hexagons and yields scalloped edges, known as “armchair” edges. Between these two extremes are a variety of chiral vectors describing edges stepped on the nanoscale, in which, for example, after every few hexagons a zigzag segment is added at an angle.

These subtle differences in edge structure have been predicted to produce measurably different physical properties, which potentially could be exploited in new graphene applications. Steven Louie of UC Berkeley and Berkeley Lab’s MSD was the research team’s theorist; with the help of postdoc Oleg Yazyev, Louie calculated the expected outcomes, which were then tested against experiment.

Chenggang Tao of MSD and UCB led a team of graduate students in performing scanning tunneling microscopy (STM) of the nanoribbons on a gold substrate, which resolved the positions of individual atoms in the graphene nanoribbons. The team looked at more than 150 high-quality nanoribbons with different chiralities, all of which showed an unexpected feature, a regular raised border near their edges forming a hump or bevel. Once this was established as a real edge feature – not the artifact of a folded ribbon or a flattened nanotube – the chirality and electronic properties of well-ordered nanoribbon edges could be measured with confidence, and the edge regions theoretically modeled.

Electronics at the edge

“Two-dimensional graphene sheets are remarkable in how freely electrons move through them, including the fact that there’s no band gap,” Crommie says.

“Nanoribbons are different: electrons can become trapped in narrow channels along the nanoribbon edges. These edge-states are one-dimensional, but the electrons on one edge can still interact with the edge electrons on the other side, which causes an energy gap to open up.”

Using an STM in spectroscopy mode (STS), the team measured electronic density changes as an STM tip was moved from a nanoribbon edge inward toward its interior. Nanoribbons of different widths were examined in this way. The researchers discovered that electrons are confined to the edge of the nanoribbons, and that these nanoribbon-edge electrons exhibit a pronounced splitting in their energy levels.

“In the quantum world, electrons can be described as waves in addition to being particles,” Crommie notes. He says one way to picture how different edge states arise is to imagine an electron wave that fills the length of the ribbon and diffracts off the atoms near its edge. The diffraction patterns resemble water waves coming through slits in a barrier.

For nanoribbons with an armchair edge, the diffraction pattern spans the full width of the nanoribbon; the resulting electron states are quantized in energy and extend spatially throughout the entire nanoribbon. For nanoribbons with a zigzag edge, however, the situation is different. Here diffraction from edge atoms leads to destructive interference, causing the electron states to localize near the nanoribbon edges. Their amplitude is greatly reduced in the interior.

The energy of the electron, the width of the nanoribbon, and the chirality of its edges all naturally affect the nature and strength of these nanoribbon electronic states, an indication of the many ways the electronic properties of nanoribbons can be tuned and modified.

Says Crommie, “The optimist says, ‘Wow, look at all the ways we can control these states – this might allow a whole new technology!’ The pessimist says, ‘Uh-oh, look at all the things that can disturb a nanoribbon’s behavior – how are we ever going to achieve reproducibility on the atomic scale?’”

Crommie himself declares that “meeting this challenge is a big reason for why we do research. Nanoribbons have the potential to form exciting new electronic, magnetic, and optical devices at the nanoscale. We might imagine photovoltaic applications, where absorbed light leads to useful charge separation at nanoribbon edges. We might also imagine spintronics applications, where using a side-gate geometry would allow control of the spin polarization of electrons at a nanoribbon’s edge.”

Although getting there won’t be simple — “The edges have to be controlled,” Crommie emphasizes — “what we’ve shown is that it’s possible to make nanoribbons with good edges and that they do, indeed, have characteristic edge states similar to what theorists had expected. This opens a whole new area of future research involving the control and characterization of graphene edges in different nanoscale geometries.”