Mohammad Ali Mohammad | University of Alberta (original) (raw)

Papers by Mohammad Ali Mohammad

Surface acoustic wave (SAW) transducers are a well-established component used in numerous sensors... more Surface acoustic wave (SAW) transducers are a well-established component used in numerous sensors, communications, and electronics devices. In this work, the authors report a systematic study of 320–800 nm period lithium niobate SAW interdigitated transducers (IDTs) corresponding to resonant frequencies in the 4–12 GHz range. An optimized SAW design and a nanofabrication process flow were developed, which enabled superior device performance in terms of frequency, signal losses, and electromagnetic coupling. The influence of the device alignment on the substrate crystal planes, in addition to
the IDT period and electrode design, is found to have a significant impact on various process metrics. As an example, two identical SAW transducers fabricated perpendicular to each other may have a resonant frequency difference approaching 1 GHz, for the same harmonic mode. These and other trends are presented and discussed.

Microelectronic Engineering, Dec 5, 2010

In this work, we report the fabrication of sub-10 nm wide, doubly-clamped silicon carbon nitride ... more In this work, we report the fabrication of sub-10 nm wide, doubly-clamped silicon carbon nitride (SiCN) resonators of up to 5 μm lengths. An existing resonator fabrication process has undergone a major improvement through the use of a single hydrogen silsesquioxane (HSQ) masking layer for SiCN patterned using electron beam lithography. Novel development strategies, comprising hot development and HF-trimming development, were also used. The crucial role of post-exposure resist processing in improving the resonator resolution and uniformity was demonstrated. Application of the optimized lithographic process has allowed us to claim the narrowest suspended bridge structures of several microns in length achieved to date.

Japanese Journal of Applied Physics, Jun 20, 2012

ZEP brand electron beam resists are well-known for their high sensitivity and etch durability. Th... more ZEP brand electron beam resists are well-known for their high sensitivity and etch durability. The various performance metrics such as sensitivity, contrast, and resolution of ZEP resist depend strongly on the development process. In this work, we investigate the development of ZEP-520 resist through contrast curves, dense gratings, and surface roughness measurements using three different classes of developer systems of varying solvation strength, ZED-N50, methyl isobutyl ketone (MIBK) : isopropyl alcohol (IPA) 1:3, and IPA : H2O 7:3, at the ambient temperature (22 °C) and cold (-15 °C) development conditions. In order to provide a deeper insight into the ZEP development process, we propose a novel kinetic model of dissolution for ZEP, and develop an efficient analytical method that allows determining the microscopic parameters of ZEP dissolution based on experimental contrast curves. We also observe experimentally and characterize the negative tone behavior of ZEP for dense grating patterning and compare its performance with positive tone behavior.

Journal of Vacuum Science & Technology B, Nov 9, 2012

Exposure of polymethylmethacrylate (PMMA) during electron beam lithography (EBL) produces small p... more Exposure of polymethylmethacrylate (PMMA) during electron beam lithography (EBL) produces small polymer fragments that dissolve rapidly during the development process. The resist dissolution behavior varies greatly depending on the nature of the developer (solvent) and therefore influences the selection of the EBL parameters, such as dose (sensitivity). A molecular scale examination of the development process is necessary to elucidate the resist–developer interaction mechanisms. In this work, the authors investigate the interaction of short PMMA chains (containing up to 10 MMA units) with common developer components methyl isobutyl ketone (MIBK) and isopropyl alcohol (IPA). For this purpose, the authors conduct molecular dynamics simulations using the Accelrys Materials Studio package. The simulation results were used to characterize the mixtures in the spirit of the Flory–Huggins theory of polymers and also to extract the diffusivities. The authors found that the behavior of PMMA fragments differed considerably in MIBK as compared with IPA. PMMA fragments containing more than three monomers exhibit stronger attractive interaction with MIBK. For all fragment sizes simulated, the diffusivity of PMMA fragments is 60–160% higher in MIBK as well. Similarly, the authors observed differences in the gyration radii. The authors conclude that the kinetic factor seems to be more significant as compared to affinity factor when accounting for differences in exposure sensitivities due to developer selection.

Journal of Vacuum Science & Technology B, Sep 22, 2011

A modern alternative to the positive-tone PMMA resist is the ZEP 520A (Nippon Zeon) brand co-poly... more A modern alternative to the positive-tone PMMA resist is the ZEP 520A (Nippon Zeon) brand co-polymer resist, which offers a higher sensitivity and etch durability for electron beam lithography. However, the molecular mechanisms are not entirely understood, and the relative performance of two resists for various process conditions of nanofabrication is not readily predictable. The authors report a thorough experimental comparison of the performance of PMMA 950k and ZEP 520A resists in MIBK:IPA, ZED, and IPA:water developers. Interestingly, ZEP resist performance was found to depend significantly on the developer. ZED developer increases the sensitivity, whereas IPA:water optimizes line edge roughness and conceivably the resolution at the expense of sensitivity. The authors also describe two alternative numerical models, one assuming an enhancement of the main chain scission in ZEP as a result of electronic excitations in side groups, and another without such enhancement. In the second case, the differences in ZEP and PMMA resists performance are attributed to their different interaction with the developers. Using both approaches, the authors parameterize the respective models of ZEP development by fitting numerical results to the experimental resist morphologies, and analyze the outcomes.

Nanoscale Research Letters, Mar 27, 2013

A detailed process characterization of SML electron beam resist for high-aspect-ratio nanopattern... more A detailed process characterization of SML electron beam resist for high-aspect-ratio nanopatterning at high sensitivity is presented. SML contrast curves were generated for methyl isobutyl ketone (MIBK), MIBK/isopropyl alcohol (IPA) (1:3), IPA/water (7:3), n-amyl acetate, xylene, and xylene/methanol (3:1) developers. Using IPA/water developer, the sensitivity of SML was improved considerably and found to be comparable to benchmark polymethylmethacrylate (PMMA) resist without affecting the aspect ratio performance. Employing 30-keV exposures and ultrasonic IPA/water development, an aspect ratio of 9:1 in 50-nm half-pitch dense grating patterns was achieved representing a greater than two times improvement over PMMA. Through demonstration of 25-nm lift-off features, the pattern transfer performance of SML is also addressed.

Journal of Vacuum Science & Technology B, Apr 27, 2007

The authors report a systematic investigation, both by experiment and by numerical modeling, of r... more The authors report a systematic investigation, both by experiment and by numerical modeling, of resolution limits for dense nanoscale gratings fabricated in polymethylmethacrylate through low-energy electron-beam lithography (EBL) using 10 keV electrons. In particular, they have studied the resist morphologies that develop in various exposure regimes for gratings with periods from 20 to 100 nm and categorized the potential sources of resist damage in such gratings. Through comparison of their experimental and numerical results, they have elucidated the major mechanisms that limit the EBL process resolution at the stages of exposure and resist development. The authors have also suggested semiempirical criteria to predict the risk of resist damage when fabricating dense nanostructures.

The continuous tuning of the emission spectrum of a single light-emitting diode (LED) by an exter... more The continuous tuning of the emission spectrum of a single light-emitting diode (LED) by an external electrical bias is of great technological significance as a crucial property in high-quality displays, yet this capability has not been demonstrated in existing LEDs. Graphene, a tunable optical platform, is a promising medium to achieve this goal. Here we demonstrate a bright spectrally tunable electroluminescence from blue (~450 nm) to red (~750 nm) at the graphene oxide/reduced-graphene oxide interface. We explain the electroluminescence results from the recombination of Poole–Frenkel emission ionized electrons at the localized energy levels arising from semi-reduced graphene oxide, and holes from the top of the π band. Tuning of the emission wavelength is achieved by gate modulation of the participating localized energy levels. Our demonstration of current-driven tunable LEDs not only represents a method for emission wavelength tuning but also may find applications in high-quality displays.

Journal of Vacuum Science & Technology B, Sep 9, 2011

The authors report an approach that has potential to fabricate dense structures without liquid de... more The authors report an approach that has potential to fabricate dense structures without liquid development. Two kinds of positive tone electron beam resist, 950k PMMA and ZEP 520A (Nippon Zeon), were studied for their properties and behaviors while subjecting them to exposure, thermal development, and reactive ion etching. So far, we have successfully patterned 70 nm half-pitch gratings in both 950k PMMA and ZEP 520A without liquid development.

Microelectronic Engineering, Nov 14, 2009

In this work, we outline our extensive study of nanostructures with 15–35 nm lateral dimensions f... more In this work, we outline our extensive study of nanostructures with 15–35 nm lateral dimensions fabricated in PMMA employing 3–30 keV electrons. We have analysed the impact of the exposure voltage and dose, as well as development time and temperature, on the 3D morphology and quality of the nanoscale gratings in PMMA. We demonstrate that, in addition to the exposure conditions that are routinely optimized in standard EBL techniques, post-exposure resist processing is also a crucial factor and should be co-optimized when fabricating dense nanopatterns in the moderate to low voltage regimes. We analyze the potential of employing low-voltage exposures combined with cold development, and discuss factors affecting resolution and sensitivity of EBL at the nanoscale.

Journal of Vacuum Science & Technology B, Jan 20, 2010

The authors report a systematic experimental study of dense nanostructures in polymethylmethacryl... more The authors report a systematic experimental study of dense nanostructures in polymethylmethacrylate (PMMA) created by low-energy electron beam lithography (EBL) with varying duration and temperature of the resist dissolution. They observe that decreasing the development temperature not only yields the widest favorable exposure dose regimes but also requires highest exposure doses to fabricate dense nanopatterns. They interpret the observed interdependence of the exposure doses and the development temperatures in terms of a simple kinetic model describing the diffusion mobility of fragments in exposed PMMA during dissolution and discuss the corresponding molecular mechanisms that determine the resolution and sensitivity of EBL nanofabrication.

Resonant glassy nanostrings have been employed for the detection of biomolecules. These devices o... more Resonant glassy nanostrings have been employed for the detection of biomolecules. These devices offer high sensitivity and amenability to large array integration and multiplexed assays. Such a concept has however been impaired by the lack of stable and biocompatible linker chemistries. Diazonium salt reduction-induced aryl grafting is an aqueous-based process providing strong chemical adhesion. In this work, diazonium-based linker chemistry was performed for the first time on glassy nanostrings, which enabled the bio-functionalization of such devices. Large arrays of nanostrings with ultra-narrow widths down to 10 nm were fabricated employing electron beam lithography. Diazonium modification was first developed on SiCN surfaces and validated by X-ray photoelectron spectroscopy. Similarly modified nanostrings were then covalently functionalized with anti-rabbit IgG as a molecular probe. Specific enumeration of rabbit IgG was successfully performed through observation of downshifts of resonant frequencies. The specificity of this enumeration was confirmed through proper negative control experiments. Helium ion microscopy further verified the successful functionalization of nanostrings.

Journal of Vacuum Science & Technology B, Dec 2, 2010

The authors report the nanomachining of sub-20-nm wide doubly clamped silicon carbon nitride reso... more The authors report the nanomachining of sub-20-nm wide doubly clamped silicon carbon nitride resonators using low keV electron beam lithography with polymethyl methacrylate resist and cold development. Methodologies are developed for precisely controlling the resonator widths in the ultranarrow regime of 11–20 nm. Resonators with lengths of 1–20 μm and widths of 16–280 nm are characterized at room temperature in vacuum using piezoelectric actuation and optical interferometry. Clamping and surface losses are identified as the dominant energy loss mechanisms for a range of resonator widths. The resonator clamping points are optimized using an original electron beam lithography simulator. Various alternative clamping point designs are also modeled and fabricated in order to reduce the clamping losses.

Biosensors and Bioelectronics, Apr 15, 2015

An atomic resolution ultra-high sensitivity surface acoustic wave (SAW) biosensor for DNA sequenc... more An atomic resolution ultra-high sensitivity surface acoustic wave (SAW) biosensor for DNA sequences and cells detection is proposed. Interdigitated transducers (IDTs) fabricated on LiNbO3 substrate achieve a high quality factor (Q) of over 4000 at a frequency of 6.4GHz (third-order harmonic mode) using an optimized design and process. The biosensor shows excellent linear responses to target DNA in the range from 1μg/ml to 1ng/ml with a high sensitivity of 6.7×10-16g/cm2/Hz, hence the difference of a single hybridized DNA base can also be distinguished. With such a high mass resolution, the biosensor is capable of quantitative detection of living cancer cells. The frequency responses of single mouse mammary adenocarcinoma (EMT6) cell and mouse fibroblast (3T3) cell are studied. The interferences in the experiments show insignificant influence on the frequency shift, which verifies the high selectivity of the biosensor. The biosensor is also able to repeat the sensing ability after rough cleaning, therefore cost reduction is achieved from the recycling process in practical applications. The detection limit is defined from the noise analysis of the device, atomic resolution is realized according to the calculation, thereby initiating a potential tool for high-precision medical diagnoses and phenomena observation at the atomic-level.

Scientific Reports

Pressure sensors are a key component in electronic skin (e-skin) sensing systems. Most reported r... more Pressure sensors are a key component in electronic skin (e-skin) sensing systems. Most reported resistive pressure sensors have a high sensitivity at low pressures (<5 kPa) to enable ultra-sensitive detection. However, the sensitivity drops significantly at high pressures (>5 kPa), which is inadequate for practical applications. For example, actions like a gentle touch and object manipulation have pressures below 10 kPa, and 10–100 kPa, respectively. Maintaining a high sensitivity in a wide pressure range is in great demand. Here, a flexible, wide range and ultra-sensitive resistive pressure sensor with a foam-like structure based on laser-scribed graphene (LSG) is demonstrated. Benefitting from the large spacing between graphene layers and the unique v-shaped microstructure of the LSG, the sensitivity of the pressure sensor is as high as 0.96 kPa−1 in a wide pressure range (0 ~ 50 kPa). Considering both sensitivity and pressure sensing range, the pressure sensor developed in this work is the best among all reported pressure sensors to date. A model of the LSG pressure sensor is also established, which agrees well with the experimental results. This work indicates that laser scribed flexible graphene pressure sensors could be widely used for artificial e-skin, medical-sensing, bio-sensing and many other areas.

A resistive random access memory (RRAM) device with a tunable switching window is demonstrated fo... more A resistive random access memory (RRAM) device with a tunable switching window is demonstrated for the first time. The SET voltage can be continuously tuned from 0.27 to 4.5 V by electrical gating from −10 to +35 V. The gate-controlled bilayer graphene-electrode RRAM can function as 1D1R and potentially increase the RRAM density.

RSC Advances, Feb 2, 2015

Graphene is flexible and transparent with one-atom layer thickness, and is a novel building block... more Graphene is flexible and transparent with one-atom layer thickness, and is a novel building block with potential applications in future portable devices. Herein a flexible, transparent and ultrathin earphone based on single-layer graphene (SLG) is reported. The SLG earphone operates in the frequency range of 20 Hz to 200 kHz and has a highest sound pressure level (SPL) of 70 dB at a 1 cm distance. The SPLs emitted from one to six layers of stacked SLG are compared. It is observed that the SPL decreases with an increasing number of stacked layers. The SLG earphone, which is packaged with a commercial earphone casing, can play music clearly. Compared with a conventional earphone, the SLG earphone has a broader frequency response and a lower fluctuation. Testing results in both time and frequency domains show a frequency doubling effect, which indicates that the working principle is based on the electro-thermoacoustic (ETA) effect. As the SLG earphone operates in both the audible and ultrasonic frequency range, it can be used for a wide variety of applications.

Piezoelectric materials used in the development of nanoscale mechanical sensors, actuators and en... more Piezoelectric materials used in the development of nanoscale mechanical sensors, actuators and energy harvesters have received much attention. More recently, devices made of graphene are of particular interest because of graphene’s intriguing electronic and mechanical properties. Intrinsic graphene has long been considered devoid of the piezoelectric effect, although flexoelectricity has been exploited to demonstrate piezoelectricity in functionalized graphene and graphene nanoribbons. The perceived lack of this property has restricted graphene’s use in nanoelectromechanical systems (NEMS) for electromechanical coupling purposes. Here an unprecedented two-dimensional (2D) piezoelectric effect on a strained/unstrained graphene junction is reported. In stark contrast to the bulk piezoelectric effect that results from the occurrence of electric dipole moments in solids, the 2D piezoelectric effect arises from the charge transfer along a work function gradient introduced by the biaxial-strain-engineered band structure. The observed effect, termed the band-piezoelectric effect, exhibits an enormous magnitude due to the ultrathin structure of graphene. On the basis of the band-piezoelectric effect, a graphene nanogenerator and a pressure gauge were fabricated. The results not only provide a versatile NEMS platform for sensing, actuating and energy harvesting, but also pave the way for efficiently modulating graphene via strain engineering.

Chinese Physics Letters, Dec 2014

We report a multiple resonant mode film bulk acoustic resonator (FBAR) with different AlN film th... more We report a multiple resonant mode film bulk acoustic resonator (FBAR) with different AlN film thicknesses of 605 nm, 640 nm and 680 nm. With the tilted c-axis orientation of the AlN piezoelectric film providing polarization vertical to the c-axis, acoustic wave resonant peaks have been observed for both the thickness shear modes (TSM 0th, TSM 1st, TSM 2nd) and the thickness extension modes (TEM 0th, TEM 1st). The corresponding parallel resonant frequencies are around 1.60 GHz, 2.41 GHz, 3.45 GHz, 2.75 GHz and 4.10 GHz, respectively. The latter two TEM modes also have good quality factors, and high equivalent electromechanical coupling coefficients Keff2 of 647, 3.13% and 113, 6.23%, respectively. By etching the 1.8 μm silicon sacrificial layer, the air gap FBAR devices have been fabricated in an easier and cleaner way resulting in a low insertion loss of −2.2 dB. The overall device structure of the top electrode/AlN film/bottom electrode on SiO2/silicon-on-insulator (SOI) substrate potentially enables CMOS compatibility. These multiple resonant mode FBAR devices will promote the integration of multi-band filters on a single chip. Improvements of the fabrication process, the influence of different AlN film thicknesses and theoretical analyses of the coexistence of multiple resonant modes are presented.

Scientific Reports, Aug 11, 2014

Recently, two-dimensional materials such as molybdenum disulphide (MoS2) have been demonstrated t... more Recently, two-dimensional materials such as molybdenum disulphide (MoS2) have been demonstrated to realize field effect transistors (FET) with a large current on-off ratio. However, the carrier mobility in backgate MoS2 FET is rather low (typically 0.5–20 cm2/V·s). Here, we report a novel field-effect Schottky barrier transistors (FESBT) based on graphene-MoS2 heterojunction (GMH), where the characteristics of high mobility from graphene and high on-off ratio from MoS2 are properly balanced in the novel transistors. Large modulation on the device current (on/off ratio of 10^5) is achieved by adjusting the backgate (through 300 nm SiO2) voltage to modulate the graphene-MoS2 Schottky barrier. Moreover, the field effective mobility of the FESBT is up to 58.7 cm2/V·s. Our theoretical analysis shows that if the thickness of oxide is further reduced, a subthreshold swing (SS) of 40 mV/decade can be maintained within three orders of drain current at room temperature. This provides an opportunity to overcome the limitation of 60 mV/decade for conventional CMOS devices. The FESBT implemented with a high on-off ratio, a relatively high mobility and a low subthreshold promises low-voltage and low-power applications for future electronics.

Microelectronic Engineering, Dec 5, 2010

Japanese Journal of Applied Physics, Jun 20, 2012

Journal of Vacuum Science & Technology B, Nov 9, 2012

Journal of Vacuum Science & Technology B, Sep 22, 2011

Nanoscale Research Letters, Mar 27, 2013

Journal of Vacuum Science & Technology B, Apr 27, 2007

Journal of Vacuum Science & Technology B, Sep 9, 2011

Microelectronic Engineering, Nov 14, 2009

Journal of Vacuum Science & Technology B, Jan 20, 2010

Journal of Vacuum Science & Technology B, Dec 2, 2010

Biosensors and Bioelectronics, Apr 15, 2015

Scientific Reports

RSC Advances, Feb 2, 2015

Chinese Physics Letters, Dec 2014

Scientific Reports, Aug 11, 2014

Nanofabrication, 2012

Electron Beam Lithography (EBL) is a fundamental technique of nanofabrication, allowing not only ... more Electron Beam Lithography (EBL) is a fundamental technique of nanofabrication, allowing not only the direct writing of structures down to sub-10 nm dimensions, but also enabling high volume nanoscale patterning technologies such as (DUV and EUV) optical lithography and nanoimprint lithography through the formation of masks and templates. This chapter summarizes the key principles of EBL and explores some of the complex interactions between relevant parameters and their effects on the quality of the resulting lithographic structures. The use of low energy exposure and cold development is discussed, along with their impacts on processing windows. Applications of EBL are explored for the fabrication of very small isolated bridge structures and for high density master masks for nanoimprint lithography. Strategies for using both positive and negative tone resists are explored.

2015 IEEE International Electron Devices Meeting (IEDM) Proceedings

We report lithium niobate (LiNbO3) surface acoustic wave (SAW) transducers with the smallest line... more We report lithium niobate (LiNbO3) surface acoustic wave (SAW) transducers with the smallest linewidth and the highest resonant frequency. A record 30 nm wide, 200 nm period nanofabricated metallic structure on non-conductive LiNbO3 enables a frequency exceeding 14 GHz. At higher frequencies, greater data throughputs and improved sensor sensitivity are enabled. A systematic study of SAW devices from lamda =200-800 nm is presented taking into account crystal orientation, device design, patterning strategies, and resonant modes. The device performance metrics such as frequency, Q-factor, insertion loss, and coupling coefficient are measured in detail and these metrics are also found to exceed the results of other recent state-of-the-art devices, when compared.

Proceedings of ISCAS (International Symposium on Circuits and Systems), Jul 4, 2011

Previous attempts have been devoted to mimic biological vision intelligence at the architectural ... more Previous attempts have been devoted to mimic biological vision intelligence at the architectural system level. In this paper, a novel imitation of biological visual system intelligence is suggested, at the device level with the introduction of novel photodiode morphology. The proposed bio-inspired nanorod photodiode puts the depletion region length on the path of the incident photon instead of on its width, as the case is with the planar photodiodes. The depletion region has a revolving volume to increase the photodiode responsivity, and thus its photosensitivity. In addition, it can virtually boost the pixel fill factor (FF) above the 100% classical limit due to decoupling of its vertical sensing area from its limited planar circuitry area. Furthermore, the suggested nanorod photodiode photosensitivity is analytically proven to be higher than that of the planar photodiode. We also show semi-empirically that the responsivity of the suggested device varies linearly with its height; this important feature has been confirmed using Sentaurus simulation. The proposed nano-photorod is believed to meet the increasingly stringent High-Resolution-Low-Light (HRLL) detection requirements of the camera-phone and biomedical imaging markets.

Proceedings of NEWCAS (New Circuits and Systems Conference), Oct 11, 2012

In this paper, nanopillars with heights of 1μm to 5μm and widths of 250nm to 500nm have been fabr... more In this paper, nanopillars with heights of 1μm to 5μm and widths of 250nm to 500nm have been fabricated with a near room temperature etching process. The nanopillars were achieved with a continuous deep reactive ion etching technique and utilizing PMMA (polymethylmethacrylate) and Chromium as masking layers. As opposed to the conventional Bosch process, the usage of the unswitched deep reactive ion etching technique resulted in nanopillars with smooth sidewalls with a measured surface roughness of less than 40nm. Moreover, undercut was nonexistent in the nanopillars. The proposed fabrication method achieves etch rates four times faster when compared to the state-of-the-art, leading to higher throughput and more vertical side walls. The fabrication of the nanopillars was carried out keeping the CMOS process in mind to ultimately obtain a CMOS-compatible process. This work serves as an initial step in the ultimate objective of integrating photo-sensors based on these nanopillars seamlessly along with the controlling transistors to build a complete bio-inspired smart CMOS image sensor on the same wafer.

Electron Devices Meeting (IEDM), 2014 IEEE International

We demonstrate a novel flexible and transparent earphone based on single-layer graphene (SLG) for... more We demonstrate a novel flexible and transparent earphone based on single-layer graphene (SLG) for the first time. The SLG earphone operates in the frequency range of 20 Hz to 200 kHz and has a highest sound pressure level (SPL) of 70 dB with a 1 W input power. The SPL emitted from one to six layers of stacked SLG are compared. It is observed that the SPL decreases with an increasing number of stacked layers. The SLG earphone is packaged into a commercial earphone casing and can play music. Compared with a conventional earphone, the SLG earphone has a broader frequency response and a lower fluctuation. Testing results in both time- and frequency-domains show a frequency doubling effect, which indicates that the working principle is based on the electro-thermoacoustic (ETA) effect. As the SLG earphone operates in both the audible and ultrasonic frequency range, it can be used for a wide variety of applications, including for interspecies communication.

Electron Devices Meeting (IEDM), 2014 IEEE International

We demonstrate a flexible, ultra-sensitive resistive pressure sensor based on the foam-like struc... more We demonstrate a flexible, ultra-sensitive resistive pressure sensor based on the foam-like structure of laser-scribed graphene (LSG). Benefitting from the unique microstructure of the LSG, the sensitivity of the pressure sensor is as high as 0.96 kPa-1 in the low pressure range (0~50 kPa), which is the highest among all reported graphene-based pressure sensors. Moreover, the sensitivity in the high pressure range (50~113 kPa) is 0.005 kPa-1. The response of the pressure sensor is highly stable up to 100 cycles with excellent performance. Our pressure sensor can meet the needs of specific applications, for example, high sensitivity for low-pressure applications and low sensitivity for high deformation applications. Moreover, the laser-scribing technology could enable large-scale production of the LSG pressure sensor with low cost in ~20 minutes. Our work indicates that laser scribed flexible graphene pressure sensors could be widely used for artificial electronic skin (e-skin), medical-sensing, bio-sensing and many other areas.

MRS Proceedings 2015

Nanogenerators (NGs) have great potential to solve the problems of energy depletion and environme... more Nanogenerators (NGs) have great potential to solve the problems of energy depletion and environmental pollution. Here, two types of flexible nanogenerators (FNGs) based on graphene oxide (GO) and multiwall carbon nanotubes (MW-CNTs) are presented. The peak output voltage and current of GO based FNG reached up to 2 V and 30 nA, respectively, under 15 N force at 1 Hz. Moreover, the output voltage could be improved to 34.4 V when the frequency was increased to 10 Hz. It was also found the output voltage increased from 0.1 V to 2.0 V using a released GO structure. The other FNG was made by MW-CNTs mixed with ZnO nanoparticles (NPs). Its output voltage and power reached up to 7.5 V and 18.75 mW, respectively, which is much larger than that of bare ZnO based FNG. Furthermore, a peak voltage of 30 V could be gained by stamping one's foot on the FNG. Finally, a modified NG was fabricated using four springs and two flexible layers. As a result, the voltage and power reached up to 9 V and 27mW, respectively. These works may bring out broad applications in energy harvesting.

Scientific reports, 2014

Recently, two-dimensional materials such as molybdenum disulphide (MoS2) have been demonstrated t... more Recently, two-dimensional materials such as molybdenum disulphide (MoS2) have been demonstrated to realize field effect transistors (FET) with a large current on-off ratio. However, the carrier mobility in backgate MoS2 FET is rather low (typically 0.5-20 cm(2)/V · s). Here, we report a novel field-effect Schottky barrier transistors (FESBT) based on graphene-MoS2 heterojunction (GMH), where the characteristics of high mobility from graphene and high on-off ratio from MoS2 are properly balanced in the novel transistors. Large modulation on the device current (on/off ratio of 10(5)) is achieved by adjusting the backgate (through 300 nm SiO2) voltage to modulate the graphene-MoS2 Schottky barrier. Moreover, the field effective mobility of the FESBT is up to 58.7 cm(2)/V · s. Our theoretical analysis shows that if the thickness of oxide is further reduced, a subthreshold swing (SS) of 40 mV/decade can be maintained within three orders of drain current at room temperature. This provides...