The high mobility and two-dimensional nature of graphene make it an attractive material for field-effect transistors. This thesis reports on work that both demonstrates the usefulness of graphene for applications – a graphene-based non-volatile memory technology in Chapter 3 and a graphite-graphite oxide field effect transistor in Chapter 4 – and as a building block for future physics experiments in the future. shape of the graphene nanogap (also in Chapter 3) and the graphite oxide sheet, a versatile insulating fabric on the nanoscale (again Chapter 4).
Electronic properties of graphene
1.2) The hexagonal Brillouin zone, shown in Figure 1.1b, has two inequal anglesKandK1placed with respect toΓat. Comparing this energy scale to the area of overlapping curves in Figure 1.3 supports the effectiveness of the energy efficient approach.
Conductance of a point contact
Right-moving electrons contribute to I and left-moving electrons contribute to I. c) Occupancy functions for electrons injected into the channel. Assuming that the channel is a ballistic conductor, except for the transfer coefficient τ ¤ 1, the right-moving electrons produce an apparent current.
Introduction
Graphene deposition
Sometimes graphene's strong interaction with the SiO2 surface [20, 21] causes sheets to be "pulled out" from the bulk [22]. Both the drawing method and a variation of the mechanical peeling method (sometimes called the modified tape method) were used in this work.
Graphite oxidation
Processing
50 nm pixel size of the alignment windows, it's worth thinking about the process in more detail to be prepared in case problems arise (and maybe have something to think about while the SEM is being written). Tm varies slightly from the identity matrix depending on the calibration of the microscope used to measure the existing features.
Electron transport measurements
CurrentI is measured by a DL Instruments current preamplifier which outputs a voltage to be read on one of the DAQ card's input ports. The DC top-gate and back-gate voltages VtgandVbg are, as before, provided by the analog outputs of the DAQ card.
Future work
Introduction
As an example of possible applications, a prototype circuit was built to demonstrate information storage based on the combination of two or more switches using ranking, where information is stored by relative device conductance magnitudes in a memory cell. Such nonvolatile, robust graphene-based atomic switches, suitable for CMOS integration as well as monolithic integration with graphene electronics, hold promise as components for atomic-scale devices.
Device fabrication
Measurements
To further investigate this phenomenon, a histogram of the conductance was collected from 1000 cycles, shown in Figure 3.4b. The top panel of Figure 3.5a shows a histogram of the measured dtatV = 2.8 V. The peak att200 ms indicates the most likely time scale where switching occurs.
Carbon chain model
The distribution Pptq for the total waiting time would then be given by the convolution of the waiting time for an n-step Poisson process with rateΓ1, known as the Erlang distribution, and a Poisson process with rateΓ2. The quality of the fit was not sensitive to the precise value of n, indicating significant uncertainty in the value of Γ1.
Application to logic circuits
The relative scales are adjusted for clarity given the different bin sizes of the two histograms (area as probability density is not directly comparable). A schematic view of the unit circuit is shown in Figure 3.7. The logical value X of the cell is defined by the relative conductivity of the two units:. Thus, the logic state of the RCC has been switched from 1 to 0 and back to 1 again.
The stability of the stored information is shown in Figure 3.8b, which shows the voltage on the two outputs read continuously for 24 hours, with no voltage applied to the write lines.
Future work
Introduction
Comparing the back and top gate efficiencies yields an estimated relative dielectric constant approximately equal to SiO2, κ 4.3. The successful operation of these devices demonstrates the utility of graphite oxide as a multilayer insulator for graphene circuits that could be used in many configurations. This shows the promise of GO for use in future carbon circuits that could be used for flexible electronics.
Device fabrication
No signal was measured from a current amplifier configured for nA sensitivity for a voltage bias of 0.5 V, indicating that the resistance is at least 10-100 GΩ. Figure 4.2: Images of graphene covered by graphite oxide. a) Optical micrograph under white light of several graphene sheets partially covered by graphite oxide. The graphite oxide layer (shaded green) is placed between the graphene and the metal electrode on the left. b) False color atomic force micrograph of a G-GO FET. Thermal reduction of graphite oxide results in at least partial recovery of the electrical conductivity of graphite [33].
To further investigate the effect, graphite oxide flakes were deposited on several Si/SiO2 sub- strates, which were then subjected to varying prebaking temperature and time before simulated electron-beam lithography.
Dielectric characterization
With experience, one can establish safe limits for the top-gate voltage for any device, based only on the measured thickness (using atomic force microscopy) of the graphite oxide. Safe top-gate voltage limits are established as part of measuring any fully functional FET, and these are listed in Table 4.1. After a safe top-gate voltage range was established, the source-drain resistance of each G-GO FET was measured using low-frequency lock-in techniques while sweeping Vtg and back-gate voltage Vbg.
The graphene channel, which is contacted at one end by source bias V and by drain bias Vd, is divided into three numbered regions defined by the edges of the top gate.
Carrier mobility
Thus, the top gate can generate a carrier density of 51012 cm−2 while remaining below the graphite oxide breakdown field mentioned above, which is sufficient for applications such as analog electronics [98]. The fluctuations in conductance visible in Figure 4.9 likely arise from a combination of disorder and phase-coherent transport within the graphene [102].
Future work
Introduction
General description
Mezurit 2 operates in setup mode2 while the “Setup” page is selected and in panel mode2 while one of the “Panel” pages is selected. The terminal works in both modes and is therefore present on every page, although not all commands are available in both modes. Mezurit 2 is inspired by a program called Mezurit3 and as such the concepts of virtual channels, logging and sweeps are present in both.
Operation
The expression must be one line of Python code that defines the function used to calculate the value of the virtual channel. 9 Because the set of virtual channel values is computed sequentially at each iteration of the acquisition loop, a virtual channel function should only refer to other channels with smaller indices. The evaluation phase occurs during each iteration of the collection loop when functions{fn} are called as needed to compute a new point{Xn}.
Note that for both cases shown in Figure 5.4, it is implied that none of the other virtual channels' bin sizes are relevant.
Scripting
The gpibpm, a, ϕq command is particularly important because it allows arbitrary messages to be sent over GPIB without interfering with GPIB-based virtual channels. One common use of the scripting interface is to automate megasweeps and gigasweeps. The first takes the form of a loop in which each iteration increments one parameter using set dacpp, Xp1q and then sweeps another parameter using sweep upppq followed by catch sweepp'max', pq.
The data may be saved after each sweep using save datapϕq or simply saved manually at the end of the megasweep.
Architecture
Later, when the pseudoclosure is called, the function is called with access to the captured data. When the user updates the setting in the GUI (step 2), an event is queued which is then processed by the. Partial data structures are represented as tables. without additional data captured, in this case) and another containing the MCF key and references to "setvar mcf", the variable and the widget.
The rowϕ is then parsed and the resulting MCF key is matched against the pseudo-closure (step 6), which is invoked (step 7), whereupon the variable is securely updated (steps 8–10).
Future work
Two examples of the latter were presented in this work: one focusing on the edge of graphene as an intrinsic nanoscale object and interfacing with atomic-scale carbon structures, and another demonstrating that oxidized graphene is useful as a layered insulator. A low-temperature electron beam lithography process was developed that maintained the integrity of the graphite oxide as measured by color contrast and transport measurements. Measurements of the channel conductance as a function of both the top gate and back gate of doped silicon showed that the dielectric constant of graphite oxide, κ, is approximately 4.3.
This work demonstrates the utility of graphite oxide as a general-purpose layered insulator compatible with graphene and likely other materials.
Introduction
Operation
Examples
The output file (example normal out.DC2) can then be loaded into NPGS via an executable file that specifies a 3-step manual alignment, using layers 1–4, and a write step, using layer 5. Screen recordings of alignment and writing processes are shown in Figure A .2. Fracture mode operation is illustrated using the test pattern shown in Figure A.3, which contains a sparse network of stretch marks. The subfields are written as separate files (exfield-0.DC2, exfield-1.DC2, exfield-2.DC2 and exfield-3.DC2).
In addition, a list of X-Y stage movements is stored as a runfile snippet (exrun.txt) that describes the stitching scheme for the overall pattern, shown in Figure A.4.
Code listing
Introduction
The rear panel (not shown) contains "banana" connectors for the power supply and a fuse holder.
Specifications and performance
Operating instructions
Circuit design
Power supply noise is controlled by 1 µF bypass capacitors (C11–C28) located near the inputs and outputs of the DC/DC converters. In addition, large 4.7 µF tantalum capacitors (C1–C4) are placed near the supply terminals of the op-amps. Given that the input terminals of the DC/DC converters are isolated from their output terminals, the negative lead of the input power was intentionally not connected to amplifier ground to eliminate a possible ground loop in the measurement setup.
The TI DCP011515DB provides isolation using an 800kHz oscillator that drives an integrated transformer connected to a rectifier at the output.
Introduction
Operation
Example
Presenting the 'm' flag will maximize the number of points available, at the cost of a much longer transfer time. The output will be an AWC-compatible text file containing several lines of header information, followed by a list of voltagesVjat timestj {0, tsamp,2tsamp, ..} wardamp 1{fsamp. The output file (for example awc.txt) can then be opened in AWC and will look similar to Figure C.2.
This output file (example test.txt) can then be easily loaded by plotting software, as shown in Figure C.3.
Load impedance
Code listing
Introduction
Thermal reduction of graphite oxide
Electron transport in reduced graphite oxide
Introduction
Startup and shutdown processes
The second example script "mega2.py" takes over most of the megasweep responsibility from the user. Parameters for the deletion, such as the destination file name, step sizes, deletion limits, etc., appear at the top of the script. These values are then programmed into the cleaning tool automatically before starting the cycle.
Raman spectroscopy study of annealing-induced effects on graphene prepared by micromechanical exfoliation. Bulletin of the Korean Chemical Society.
