We have produced ultrathin epitaxial graphite films which show remarkable 2D electron gas (2DEG) behavior. The films, composed of typically 3 graphene sheets, were grown by thermal decomposition on the (0001) surface of 6H-SiC, and characterized by surface-science techniques. The low-temperature conductance spans a range of localization regimes according to the structural state (square resistance 1.5 kOhm to 225 kOhm at 4 K, with positive magnetoconductance). Low resistance samples show characteristics of weak-localization in two dimensions, from which we estimate elastic and inelastic mean free paths. At low field, the Hall resistance is linear up to 4.5 T, which is well-explained by n-type carriers of density 10^{12} cm^{-2} per graphene sheet. The most highly-ordered sample exhibits Shubnikov - de Haas oscillations which correspond to nonlinearities observed in the Hall resistance, indicating a potential new quantum Hall system. We show that the high-mobility films can be patterned via conventional lithographic techniques, and we demonstrate modulation of the film conductance using a top-gate electrode. These key elements suggest electronic device applications based on nano-patterned epitaxial graphene (NPEG), with the potential for large-scale integration.

Ultrathin epitaxial graphite: 2D electron gas properties and a route toward graphene-based nanoelectronics

Catherine Durnell Cramton (\"The Mutual Knowledge Prob- conditions, the impact of organizational change on subsequent lem and Its Consequences for Dispersed Collaboration\") is Asso- performance and survival, and the consequences of organizational ciate Professor in the School of Management at George Mason structures and dynamics for employees\' careers and the compo- University. She recieved her Ph.D. in organizational behavior from sition of organizations\' work forces. She serves on the editorial Yale University. Her research explores the contemporary chal- boards of Organization Science, Administrative Science Quar- lenges of collaboration and leadership, including distributed work, terly, and the American Sociological Review. Address: Columbia interorganizational collaboration, project team leadership, and the University, Graduate School of Business, 706 Uris Hall, New impact of technology on collaboration. Address: School of Man- York, NY 10027-6902; telephone: (212) 854-4424; e-mail: agement, Mail Stop 5F5, George Mason University, Fairfax, VA hah15@columbia.edu . 22030-4444; telephone: (703) 993-1814; fax: (202) 318-4319; John C. Henderson (\"Understanding `Strategic Learning\': e-mail: ccramton@som.gmu.edu . Linking Organizational Learning, Knowledge Management, and Sue R. Faerman (\"Understanding Interorganizational Cooper- Sensemaking\") is the Richard C. Shipley Professor of Manage- ation: Private-Public Collaboration in Regulating Financial Market ment, Chairperson of the Information Systems Department, and Innovation\") is Professor of Public Administration and Organiza- Director of the Institute for Leading in a Dynamic Economy at tional Studies and Dean of Undergraduate Studies at the University Boston University\'s School of Management. He is a noted at Albany, State University of New York. Her research focuses on researcher, consultant, and executive educator with papers appear- the paradoxical nature of leadership and organizational perfor- ing in many refereed journals. His coauthored book, Knowledge mance. She received her Ph.D. in public administration from the Engine, examines leveraging a firm\'s knowledge assets. His University at Albany, State University of New York. Address: research interests include: managing strategic partnerships, impact Office of Undergraduate Studies-LC30, University at Albany, of the mobile Internet on markets and organizations, and knowl- State University of New York, 1400 Washington Avenue, Albany, edge management. Address: Boston University School of Man- NY 12222; telephone: (518) 442-3950; fax: (518) 442-4959; e-mail: agement, 595 Commonwealth Avenue, Room 546H, Boston, MA sfaerman@uamail.albany.edu . 02215; telephone: (617) 353-6142; fax: (617) 353-1695; e-mail: James F. Fairbank (\"Emulation in Academia: Balancing Struc- jchender@bu.edu . ture and Identity\") received his Ph.D. in management and organi- Giuseppe (Joe) Labianca (\"Emulation in Academia: Balancing zation, The Pennsylvania State University and is an assistant pro- Structure and Identity\") received his Ph.D. in management and or- fessor of management in the College of Business and Economics ganization, The Pennsylvania State University, and is an assistant at West Virginia University in Morgantown, WV. His current re- professor of organization and management at Emory University\'s search interests are strategic decision making and the management Goizueta Business School in Atlanta, GA. His primary interests are of information technology. Address: College of Business and Eco- in network and cognition research at the intra- and interorganiza- nomics, P.O. Box 6025, Morgantown, WV 26506-6025; telephone: tional levels. Recent projects have investigated the antecedents and (304) 293-7937; fax: (304) 293-5652; e-mail: ifairban@wvu.edu . consequences of disliking others in one\'s social networks at work, Dennis A. Gioia (\"Emulation in Academia: Balancing Structure as well as investigating the process of emulation between organ- and Identity\") is Professor of Organizational Behavior, Department zations. He conducts both quantitative and qualitative research, and of Management and Organization, Smeal College of Business Ad- has published in the Academy of Management Journal and Orga- ministration at The Pennsylvania State University. His current nization Science. He is currently serving on the editorial review research and writing interests focus primarily on the cognitive board of the Academy of Management Journal. Address: Goizueta processes of organization members, especially the ways in which Business School, 1300 Clifton Road, Emory University, Atlanta, identity, image, and reputation are involved in organizational GA 30322; telephone: (404) 727-7662; fax: (404) 727-6663; e-mail: sensemaking, sensegiving, and change. His work has appeared joe_labianca@bus.emory.edu . in many of the top journals in the field and he has coedited two David P. McCaffrey (\"Understanding Interorganizational Co- volumes of original contributions: The Thinking Organization operation: Private-Public Collaboration in Regulating Financial and Creative Action in Organizations. Address: Smeal College of Market Innovation\") is Professor of Public Administration, Public Business, Penn State University, 403 Beam BAB, University Park, Policy, and Organizational Studies at the University at Albany, PA 16802; telephone: (814) 865-6370; fax: (814) 863-7261; e-mail: State University of New York. His research focuses on the orga- dag4@psu.edu . nizational dynamics of regulatory systems. He received his Ph.D. Heather A. Haveman (\"Organizational Environments in Flux: in sociology from the State University of New York at Stony The Impact of Regulatory Punctuations on Organizational Do- Brook. Address: Department of Public Administration and Policy, mains, CEO Succession, and Performance\") is Professor of Man- University at Albany, State University of New York, 135 Western agement at the Graduate School of Business, Columbia University. Avenue, Albany, NY 12222; e-mail: d.mccaffrey@albany.edu . She received her Ph.D. in organizational behavior and industrial Alan D. Meyer (\"Organizational Environments in Flux: The relations from the Haas School of Business Administration at the Impact of Regulatory Punctuations on Organizational Domains, University of California at Berkeley in 1990. Her research inves- CEO Succession, and Performance\") is the Charles H. Lundquist tigates organizations\' responses to shifting internal and external Professor of Entrepreneurial Management at the University of

About Authors

It is essential to control the electronic structure of graphene in order to apply graphene films for use in electrodes. We have introduced chemical dopants that modulate the electronic properties of few-layer graphene films synthesized by chemical vapor deposition. The work function, sheet carrier density, mobility, and sheet resistance of these films were systematically modulated by the reduction potential values of dopants. We further demonstrated that the power generation of a nanogenerator was strongly influenced by the choice of a graphene electrode with a modified work function. The off-current was well quenched in graphene films with high work functions (Au-doped) due to the formation of high Schottky barrier heights, whereas leakage current was observed in graphene films with low work functions (viologen-doped), due to nearly ohmic contact.

http://ma.ecsdl.org/content/MA2011-02/40/2383.full.pdf

Control of Electronic Structure of Graphene by Various Dopants and Their Effects on a Nanogenerator

In this paper the potentiality of impact avalanche transit time (IMPATT) devices based on different semiconductor materials such as GaAs, Si, InP, 4H-SiC and Wurtzite-GaN (Wz-GaN) has been explored for operation at terahertz frequencies. Drift–diffusion model is used to design double-drift region (DDR) IMPATTs based on different materials at millimeter-wave (mm-wave) and terahertz (THz) frequencies. The performance limitations of these devices are studied from the avalanche response times at different mm-wave and THz frequencies. Results show that the upper cut-off frequency limits of GaAs and Si DDR IMPATTs are 220 GHz and 0.5 THz, respectively, whereas the same for InP and 4H-SiC DDR IMPATTs is 1.0 THz. Wz-GaN DDR IMPATTs are found to be excellent candidate for generation of RF power at THz frequencies of the order of 5.0 THz with appreciable DC to RF conversion efficiency. Further, it is observed that up to 1.0 THz, 4H-SiC DDR IMPATTs excel Wz-GaN DDR IMPATTs as regards their RF power outputs. Thus, the wide bandgap semiconductors such as Wz-GaN and 4H-SiC are highly suitable materials for DDR IMPATTs at both mm-wave and THz frequency ranges.

/pdf/prospects-of-impatt-devices-based-on-wide-bandgap-2riuywl3gk.pdf

Prospects of IMPATT devices based on wide bandgap semiconductors as potential terahertz sources

A highly sensitive and selective fluorescence sensor for detection of chromium(VI) and hydrogen sulfide was developed by using silicon nanoparticles (SiNPs) as probe. The fluorescence of SiNPs was effectively quenched by Cr(VI) via the inner filter effect. Upon addition of hydrogen sulfide, the fluorescence of SiNPs was recovered due to the oxidation-reduction between Cr(VI) and H 2 S. Under optimal conditions, the wide linear response ranges were obtained over the range of 0.1–200 μM and 0.1–200 μM with the low detection limits of 28 nM and 22 nM for Cr(VI) and H 2 S, respectively. The sensing platform was successfully applied to determination of Cr(VI) and H 2 S in real samples.

On–off–on fluorescent silicon nanoparticles for recognition of chromium(VI) and hydrogen sulfide based on the inner filter effect

Potentiality of Impact Avalanche Transit Time (IMPATT) devices based on different semiconductor materials such as InP, 4H-SiC, and Wurtzite-GaN (Wz-GaN) has been explored for operation at terahertz (THz) frequencies. Drift-diffusion model is used to design double-drift region (DDR) IMPATTs based on above mentioned materials at different millimeter-wave (mm-wave) and THz frequencies and the upper cut-off frequency limits of those devices are obtained from the avalanche response times at those mm-wave and THz frequencies. Results show that the upper cut-off frequency limits of both InP and 4H-SiC DDR IMPATTs are 1.0 THz; whereas, the same is 5.0 THz in Wz-GaN DDR IMPATTs. The Wz-GaN DDR IMPATTs emerge as the most suitable devices for generation of THz frequencies due to their small avalanche response time, high DC to RF conversion ratio, and sufficiently high RF power output at THz frequencies. But, it is observed that up to 1.0 THz, 4H-SiC DDR IMPATTs excel Wz-GaN DDR IMPATTs due to their higher ou...

Potentiality of IMPATT Devices as Terahertz Source: An Avalanche Response Time-based Approach to Determine the Upper Cut-off Frequency Limits

An attempt is made in this paper to study the influence of skin depth on the parasitic series resistance of millimeter-wave IMPATT devices based on Silicon. The method is based on the concept of depletion width modulation of the device under large-signal condition. A large-signal simulation model based on non-sinusoidal voltage excitation is used for this study. The electric field snap-shots of 35 GHz Single-Drift Region (SDR) and 94 GHz Double-Drift Region (DDR) IMPATT devices are first obtained from which the series resistances are estimated by incorporating the effect of skin depth in the modeling and simulation. The series resistances of these devices are also obtained by neglecting the effect of skin depth. The values of series resistances obtained from the simulation are compared with the corresponding experimentally reported values. It is observed that the series resistance estimated by including the skin effect is in closer agreement with the experimental values as compared to that without including the same. Thus the skin effect plays an important role for determining the series resistance of IMPATT devices at millimeter-wave frequency bands.

Influence of skin effect on the series resistance of millimeter-wave IMPATT devices

The degradation of high-frequency characteristics of a 1.0-THz double-drift region (DDR) impact avalanche transit time (IMPATT) diode based on wurtzite gallium nitride (Wz-GaN), due to the influence of parasitic series resistance, has been investigated. A two-dimensional (2-D) large-signal (L-S) simulation method based on a non-sinusoidal voltage excitation (NSVE) model has been used for this purpose. A comprehensive model of series resistance has been developed by considering the influence of skin effect, and the said model has been incorporated in the 2-D L-S simulation for studying the effect of RF power output and DC to RF conversion efficiency of the device. Results indicate 24.2–35.9% reduction in power output and efficiency due to the RF power dissipation in the positive series resistance. However, the device can still deliver 191.7–202.9 mW peak RF power to the load at 1.0 THz with 8.48–6.41% conversion efficiency. GaN IMPATT diodes are capable of generating higher RF power at around 1 THz than conventional diodes, but the effect of parasitic series resistance causes havoc reduction in power output and efficiency. The nature of the parasitic resistance is studied here in the level of device fabrication and optimization, which to our knowledge is not available at present.

1.0 THz GaN IMPATT Source: Effect of Parasitic Series Resistance

The authors have developed a large-signal simulation technique extending an in-house small-signal simulation code for analyzing a 94 GHz double-drift region impact avalanche transit time device based on silicon with a non-sinusoidal voltage excitation and studied the effect of junction temperature between 300 and 550 K on the large-signal characteristics of the device for both continuous wave (CW) and pulsed modes of operation. Results show that the large-signal RF power output of the device in both CW and pulsed modes increases with the increase of voltage modulation factor up to 60%, but decreases sharply with further increase of voltage modulation factor for a particular junction temperature; while the same parameter increases with the increase of junction temperature for a particular voltage modulation factor. Heat sinks made of copper and type-IIA diamond are designed to carry out the steady-state and transient thermal analysis of the device operating in CW and pulsed modes respectively. Authors have adopted Olson's method to carry out the transient analysis of the device, which clearly establishes the superiority of type-IIA diamond over copper as the heat sink material of the device from the standpoint of the undesirable effect of frequency chirping due to thermal transients in the pulsed mode.

http://iopscience.iop.org/article/10.1088/1674-4926/34/2/024001/pdf

Aritra Acharyya

Papers

Prospects of IMPATT devices based on wide bandgap semiconductors as potential terahertz sources

Potentiality of IMPATT Devices as Terahertz Source: An Avalanche Response Time-based Approach to Determine the Upper Cut-off Frequency Limits

Influence of skin effect on the series resistance of millimeter-wave IMPATT devices

1.0 THz GaN IMPATT Source: Effect of Parasitic Series Resistance

Effect of junction temperature on the large-signal properties of a 94 GHz silicon based double-drift region impact avalanche transit time device