Collaborators

• M. Dresselhaus, EECS, MIT
• A. Chandrakasan, EECS, MIT
• T. Palacios, EECS, MIT
• V. Bulovic, EECS, MIT
• P. Jarrillo-Herrero, Physics, MIT
• M. Strano, ChemE, MIT
• K. Gleason, ChemE, MIT
• J. Zhang and Z.F.Liu, Chemistry, Peking University

Postdoctoral Associates

• D. Nezich
• K. K. Kim
• Y. M. Shi
• S. M. Kim
• S. M. Jung

Graduate Students

• K. J. Lee, Res. Asst., EECS
• M. Hofmann, Res. Asst., EECS
• A. L. Hsu, Res. Asst., EECS
• Y. C. Shin, Res. Asst., DMSE
• H. S. Park, Res. Asst., EECS
• W. J. Fang, Res. Asst., EECS
• M. S. Choi, Res. Asst., Mech E

Research Staff

• T. Y. Zeng, Research Scientist

Support Staff

• L. von Bosau, Admin. Asst. I

Publications

Ya-Ping Hsieh, Mario Hofmann, Hootan Farhat, Eduardo B. Barros, Martin Kalbac, Jing Kong**, Chi-Te Liang, Yang-Fang Chen, and Mildred S. Dresselhaus, “Chiral angle dependence of resonance window widths in (2n+m) families of single-walled carbon nanotubes,” Appl. Phys. Lett. 96, 103118, 2010.

Yumeng Shi, Ki Kang Kim, Alfonso Reina, Mario Hofmann, Lain-Jong Li, Jing Kong** “Work function engineering of graphene electrode via chemical doping,” ACS Nano, vol 4, 2689-2694, 2010.

Liming Xie, Shin G. Chou, Ajay Pande, Jayanti Pande, Jin Zhang, Mildred S. Dresselhaus, Jing Kong**, Zhongfan Liu, “Single-Walled Carbon Nanotubes Probing the Denaturation of Lysozyme”, J. of Physical Chemistry C, Vol. 114, Issue 17, pp. 7717-7720, May 2010.

Ki Kang Kim, Alfonso Reina, Yumeng, Shi, Hyesung Park, Lain-Jong Li, Young Hee Lee, Jing Kong**, “Enhancing the conductivity of transparent graphene films via doping”, Nanotechnology, Vol. 21, Issue: 28, Article Number 285205, 6 pages, July 2010).

Yumeng Shi, Christoph Hamsen, Xiaoting Jia, Ki Kang Kim, Alfonso Reina, Mario Hofmann, Allen Long Hsu, Kai Zhang, Henan Li, Zhen-Yu Juang, Mildred. S. Dresselhaus, Lain-Jong Li, and Jing Kong**, “Synthesis of Few-Layer Hexagonal Boron Nitride Thin Film by Chemical Vapor Deposition,” Nano Letters, pp. 4034-4039, September 2, 2010.

Sreekar Bhaviripudi, Xiaoting Jia, Mildred S. Dresselhaus, and Jing Kong**, “Role of Kinetic Factors in Chemical Vapor Deposition Synthesis of Uniform Large Area Graphene Using Copper Catalyst,” Nano Letters, pp. 4128–4133, September 2, 2010.

Hyesung Park, Jill A. Rowehl, Ki Kang Kim, Vladimir Bulovic, and Jing Kong, “Doped graphene electrodes for organic solar cells,” Nanotechnology, 21 505204, 6 pages, 2010.

Kyeong-Jae Lee, Masood Qazi, Jing Kong, Anantha P. Chandrakasan, “Low-Swing Signaling on Monolithically Integrated Global Graphene Interconnects,” IEEE Trans. Electron Devices, Vol .57,  Issue 12, pp. 3418-3425, Dec 2010

Thiele, S, Reina, A, Healey, P, Kedzierski, J, Wyatt, P, Hsu, PL, Keast, C, Schaefer, J, Kong, J, “Engineering polycrystalline Ni films to improve thickness uniformity of the chemical-vapor-deposition-grown graphene films”, Nanotechnology, vol 21, Art. No. 015601, 2010.

The most common substrate for processing chemical vapor deposition (CVD) graphene and highly oriented pyrolytic graphite (HOPG) has been thermally grown silicon dioxide on top of silicon. Due to optical interference, monolayer and bilayers of graphene can be easily identified using a standard optical microscope ^[1] . Furthermore, graphene is capacitively coupled to the underlying silicon, allowing for rapid electrical characterization through substrate biasing. While this substrate has proven useful for probing carrier transport in graphene as well as for developing graphene processing technology, the parasitic capacitances associated with the underlying silicon limit high-frequency performance. In this work, we have explored fabrication of graphene on purely insulating substrates, specifically sapphire. Sapphire is a common substrate used for radio frequency (RF) applications due to its low conductivity. Furthermore, it is much cheaper than other insulating substrates such as SiC or diamond.

Through recent advances in improved ohmic processing using an oxidized aluminum capping layer, we are able to fabricate high-speed field effect transistors on both silicon dioxide and sapphire ^[2] . Figure 1 shows measured RF data for a device with gate length (L_g) = 2 µm and width (W) = 25 µm on each substrate. While RF performance of graphene on silicon is slightly better after de-embedding than that of the sample on sapphire, we attribute this discrepancy to non-optimized processing conditions, possibly due to the transfer anneal or fabrication problems on transparent substrates. Fortunately, we are still able to achieve an extrinsic f_t-L_g product of 14 GHz-µm on sapphire, which to our best knowledge is the highest reported extrinsic value on CVD graphene (Figure 2 ^{[3] [4] [5] [6] [7] [8]} ). In parallel, scanning photocurrent measurements of graphene on silicon dioxide show optical sensitivities of 0.4 mA/W for large-area devices (>375 µm²). Therefore, combining our results on sapphire with optical measurements should hopefully allow for large-area arrays of high-speed graphene photodetectors.

1. X. Wang, M. Zhao, and D. D. Nolte, “Optical contrast and clarity of graphene on an arbitrary substrate,” Applied Physics Letters, vol. 95, p. 081102, 2009.
2. A. Hsu, H. Wang, K. K. Kim, J. Kong, and T. Palacios, “Impact of graphene interface quality on contact resistance and RF device performance,” submitted for publication.
3. J. S. Moon, D. Curtis, M. Hu, D. Wong, C. McGuire, P. M. Campbell, G. Jernigan, J. L. Tedesco, B. VanMil, R. Myers-Ward, C. Eddy, Jr., and D. K. Gaskill, “Epitaxial-graphene RF field-effect transistors on Si-face 6H-SiC substrates,” Electron Device Letters, vol. 30, pp. 650, 2009.
4. Y. Wu, Y. Lin, A. A. Bol, K. A. Jenkins, F. Xia, D. B. Farmer, Y. Zhu, and P. Avouris, “High-frequency, scaled graphene transistors on diamond-like carbon,” Nature, vol. 472, p. 74, 2011.
5. Y. Lin, C. Dimitrakopoulos, K.A. Jenkins, D. B. Farmer, H.Y. Chiu, A. Grill and P. Avouris, “100-GHz Transistors from wafer-scale epitaxial graphene,” Science vol. 327, p. 662, 2010.
6. Y. Lin, K. A. Jenkins, A. Valdes-Garcia, J. P. Small, D. B. Farmer and P. Avouris, “Operation of graphene transistors at gigahertz frequencies,” Nano Lett., vol. 9, p. 422, 2009.
7. L. Liao, Y. C. Lin, M. Bao, R. Cheng, J. Bai, Y. Liu, Y. Qu, K. L. Wang, Y. Huang, and X. Duan, “High-speed graphene transistors with a self-aligned nanowire gate,” Nature, vol. 467, p. 305, 2010.
8. J. Plouchart, J. Kim, J. Gross, R. Trzcinski, and K. Wu, “Scalability of SOI CMOS technology and circuit to millimeter-wave performance,” presented at CSIC Symposium 2005.

Graphene is a two-dimensional (2D) material that has attracted great interest for electronic devices since the demonstration of field effect carrier modulation in 2004 ^[1] . Its high mobility and high saturation velocity make graphene a promising material for next generation of high-frequency devices ^[2] , and its 2D geometry also makes it highly compatible with existing fabrication technology in the semiconductor industry. Furthermore, the possibility of large-scale synthesis of graphene by chemical vapor deposition (CVD) and epitaxial growth ^{[3] [4] [5]} makes graphene integrated circuits a feasible reality in the near future. Hence, it is desirable to develop a compact physical model that can enable the use of computer-aided-design software to simulate future complex circuits. In this work, we develop a compact model for the current-voltage characteristics of graphene field effect transistors (GFETs), which is based on an extension of the “virtual-source” model previously proposed for Si MOSFETs ^{[6] [7]} and is valid for both saturation and non-saturation regions of device operation (Figure 1). This virtual source model provides a simple and intuitive understanding of carrier transport in GFETs, allowing extraction of the virtual source injection velocity v_VS, a physical parameter with great technological significance for short-channel graphene transistors. With only a small set of fitting parameters, the model shows excellent agreement with experimental data (Figure 2). It is also shown that the extracted virtual source carrier injection velocity for graphene devices is much higher than in Si MOSFETs and state-of-the-art III-V HFETs with similar gate length, supporting the great potential of GFETs for high frequency applications. Future work includes extending the model for both small signal and large signal modeling of GFETs RF performance and implementation in Verilog to enable modeling of graphene circuits.

1. K. S. Novoselov, et al., “Electric field effect in atomically thin carbon films,” Science, vol. 306, pp. 666-669, Oct. 2004.
2. T. Palacios, et al. “Applications of graphene devices in RF communications,” IEEE Comm. Mag., vol. 48,  no. 6, pp. 122-128, June 2010.
3. A. Reina, et al., “Large area few-layer graphene films on arbitrary substrates by Chemical Vapor Deposition,” Nano Lett., vol. 9, no. 1, pp. 30-35, Jan. 2009.
4. X. Li, et al. “Large-area synthesis of high-quality and uniform graphene films on copper foils,” Science, vol. 324. no. 5932, June 2009.
5. C. Berger, et al. “Electronic confinement and coherence in patterned epitaxial graphene,” Science, vol. 312. no. 5777, May 2006.
6. A. Khakifirooz, O. M. Nayfeh, and D. Antoniadis “A simple semiempirical short-channel MOSFET current-voltage model dontinuous across all regions of operation and employing only physical parameters,” IEEE Trans. Electron Devices, vol. 56, no. 8, Aug. 2009.
7. D. A. Antoniadis, I. Åberg, C. N. Chleirigh, O. M. Nayfeh, A. Khakifirooz, and J. L. Hoyt, “Continuous MOSFET performance increase with device scaling: The role of strain and channel material innovation,” IBM J. Res. Develop., vol. 50, no. 4/5, pp. 363–376, July 2006.

Graphene is a one-atom-thick layer of carbon atoms arranged in a honeycomb lattice through sp² bonding ^[1] . Considered for many years an impossible goal, the isolation of graphene triggered a revolution not only among condensed-matter physicists but also among chemists and engineers, eager to take advantage of its unique properties ^[2] . The symmetry of its honeycomb lattice structure confers to graphene very unique transport properties ^[3] . For example, the carriers in graphene lose their effective mass and can be described by a Dirac-like equation instead of by the Schrödinger equation used in traditional semiconductors. This very low effective mass is responsible for a very high electron and hole mobility in excess of 100,000 cm²/Vs at room temperature ^[4] , the highest ever reported for any semiconductor. In this work, we develop novel graphene electronic devices using graphene grown by chemical vapor deposition. These new applications of graphene rely on its ambipolar transport properties to provide unique device level functionalities, which can significantly simplify the designs of many basic RF circuit blocks, such as frequency multipliers and mixers ^{[5] [6]} . The new graphene ambipolar frequency multipliers, integrated on a sapphire substrate, can operate at 16 GHz with extremely high output spectral purity (> 90%) (Figure 1) and are the fastest circuit level demonstration ever made using graphene. These Ku-band graphene frequency multipliers, made from wafer-scale graphene synthesis and fabrication processes, show the great potential of graphene-based ambipolar devices for RF and mixed-signal applications. Other applications such as graphene ambipolar mixers (Figure 2) can suppress odd order intermodulations more effectively than in traditional unipolar FET mixers [6]. These graphene devices have great potential for applications in future high performance transparent and flexible electronics.

1. K. S. Novoselov, et al., “Electric field effect in atomically thin carbon films” Science, vol. 306, pp. 666-669, Oct. 2004.
2. A. K. Geim, “Graphene: status and prospects,” Science, vol. 324, no. 5934, pp. 1530-1534, June 2009.
3. N. M. R. Peres, et al., “Electronic properties of disordered two-dimensional carbon,” Phys. Rev. B, vol. 73, pp. 125411, Mar. 2006.
4. K. Bolotin et al., “Ultrahigh electron mobility in suspended graphene,” Solid State Communications, vol. 146, pp. 351-355, June 2008.
5. H. Wang, D. Nezich, J. Kong, and T. Palacios “Graphene frequency multipliers,” IEEE Electron Device Lett., vol. ED-30, May 2009.
6. H. Wang, A. Hsu, J. Wu, J. Kong, and T. Palacios “Graphene-based ambipolar RF mixers,” IEEE Electron Device Lett., vol. ED-31, no. 9, Sept. 2010.

Thanks to its all-surface 2D structure combined with a very high carrier mobility, graphene is a very promising candidate for high sensitivity and low noise chemical sensing ^{[1] [2]} . Indeed, graphene devices can perform electrical detection for chemical sensing in a wide variety of applications, including pH monitoring in electrolytes ^{[3] [4]} ; glucose measurements in blood; and in-vitro recording of the electrical activity of living cells ^[5] .

Our group has developed the first solution-gated graphene field effect transistor (SGFET) arrays. These devices have important advantages with respect to traditional sensors, including intrinsic signal amplification, simple electronic read-out, and straightforward integration with microelectronics. Figure 1 shows a diagram of the graphene biosensor device and operating procedure. We use graphene grown by chemical vapor deposition as the active channel of these new devices. After fabricating graphene transistors with our standard technology, an SU8-based encapsulation technology has been developed to protect the contacts and wires from the electrolyte.

As seen in Figure 2.a), the characterization of our devices in a phosphate buffer solution displays high transconductances around 5 mS.mm^-1 (or 6 mS.V^-1 if normalized by the drain-source voltage). This high response highlights the very good sensitivity of our sensor compared to the conventional silicon devices, which typically have 20 times lower transconductances. The pH measurements have been performed using these devices, by recording how the Dirac point shifts with changes in pH, as shown in Figure 2.b). Sensitivity as high as 30 mV/pH have been demonstrated in this way. In addition, long term pH monitoring was shown for the first time in graphene devices, as shown in Figure 2.c). Our on-going work focuses on functionalizing graphene to demonstrate new biosensors focused on glucose detection. We are also optimizing the fabrication process to improve the sensitivity and decrease the noise of the devices.

1. M. Dankerl, M. V. Hauf, A. Lippert, L. H. Hess, S. Birner, I. D. Sharp, A. Mahmood, P. Mallet, J. Veuillen, M. Stutzmann, and J. A. Garrido, “Graphene solution-gated field-effect transistor array for sensing applications,” Adv. Funct. Mater., vol. 20, pp. 3117-3124, Sep. 2010.
2. B. Mailly Giacchetti, A. Hsu, H. Wang, K. K. Kim, J. Kong, and T. Palacios, “CVD-grown graphene solution-gated field effect transistors for pH sensing,” MRS Proceedings, vol. 1283, 2011.
3. Y. Ohno, K. Maehashi, Y. Yamashiro, and K. Matsumoto, “Electrolyte-gated graphene field-effect transistors for detecting pH and protein absorption,” Nano Lett., vol. 9, pp. 3318-3322, Sep. 2009.
4. P. K. Ang, W. Chen, A. T. Wee, and K. P. Loh, “Solution-gated epitaxial graphene as pH sensor,” J. Am. Chem. Soc., vol. 130, pp. 14392-14393, Nov. 2008.
5. T. Cohen-Karni, Q. Qing, Q. Li, Y. Fang, and C. M. Lieber, “Graphene and nanowire transistors for cellular interfaces and electrical recording,” Nano Lett., vol. 10, pp. 1098-1102, Mar. 2010.

Organic photovoltaics (OPV) has gained much attention as a possible candidate for the next generation of clean electricity due to organic semiconductors’ high absorption coefficients, light weight and flexibility, and low-cost, high throughput fabrication methods ^{[1] [2]} .  In optoelectronics devices, indium tin oxide (ITO) has been widely used as transparent conducting electrodes.  However, the need for a substitute for ITO is ever increasing due to the limited availability of indium on earth; furthermore, device issues like susceptible ion diffusion into the organic films ^[3] and mechanical brittleness ^[4] limit the applicability of ITO in OPVs. Therefore, an ITO-substitute needs to be developed with these characteristics: low-cost, mechanically robust, transparent, electrically conductive, and ultimately capable of demonstrating performance comparable to ITO-based photovoltaics.

In the past, we have synthesized graphene sheets using Ni thin film (300 nm) as a catalyst layer via atmospheric pressure chemical vapor deposition (APCVD): either single to few-layer graphene sheets or multi-layer graphene sheets (>10 layers). The sheet resistance and optical transmittance obtained from the multi-layer graphene were around 500~1000Ω/sq and 75%, respectively. We further improved the synthesis conditions using copper foil (25 µm) as metal catalyst via low-pressure chemical vapor deposition (LPCVD). This method enabled us to synthesize large area, uniform monolayer graphene (>90%) with improved electrical conductivity (400~500Ω/sq) and optical transmittance (~97%). By transferring several times, we could further improve the quality of graphene electrodes (e.g., 3-layer graphene sheet: 300-400 Ω/sq with >90% transmittance) (Figure 1). We then successfully integrated these graphene sheets into the OPV with overall performance comparable, but slightly inferior, to ITO counterparts, possibly due to the relatively higher sheet resistance. Moreover, due to the hydrophobicity of graphene’s surface, uniform coverage of PEDOT:PSS layer was challenging, which was detrimental to device success rates. Various PEDOT:PSS alternatives were investigated, and it was found that AuCl₃ doping significantly improves the graphene OPV device performances, possibly due to the improved conductivity and the work function tuning of graphene electrodes as well as the PEDOT wettability (Figure 2).

1. C. W. Tang, “2-layer organic photovoltaic cell,” Applied Physics Letters, vol. 48, pp. 183-185, Jan. 1986.
2. P. Peumans, A. Yakimov, and S. R. Forrest, “Small molecular weight organic thin-film photodetectors and solar cells,” Journal of Applied Physics, vol. 93, pp. 3693-3723, 2003.
3. A. R. Schlatmann, D. W. Floet, A. Hilberer, F. Garten, P. J. M. Smulders, T. M. Klapwijk, and G. Hadziioannou, “Indium contamination from the indium-tin-oxide electrode in polymer light-emitting diodes,” Applied Physics Letters, vol. 69, pp. 1764-1766, Sep. 1996.
4. Z. Chen, B. Cotterell, W. Wang, E. Guenther, and S. J. Chua, “A mechanical assessment of flexible optoelectronic devices,” Thin Solid Films, vol. 394, pp. 202-206, Aug. 2001.

As process technology scales, the importance of material and architectural innovation for interconnect performance will continue to increase. Graphene has attracted much interest as a replacement for copper interconnects due to its high conductivity and high current-carrying capacity ^{[1] [2] [3]} . Graphene sheets are also an attractive alternative to carbon nanotube-based interconnects as they are more compatible with conventional lithography methods. The purpose of this project is to integrate graphene devices as redundant interconnects for a low-power field-programmable gate array (FPGA). Interconnect delay is a significant portion of the delay due to multiple routing segments in an FPGA. Furthermore, global interconnects have been shown to dominate the total power consumption in FPGAs ^[4] .

In this work, we monolithically integrate graphene interconnects on a prototype CMOS chip. Large-area graphene sheets are first grown by chemical vapor deposition ^[5] and transferred onto the CMOS chip. The large graphene sheet is then lithographically patterned and etched into interconnect wires. Each end of the graphene wire is electrically connected to the underlying transmitter/receiver pair. The test chip includes an FPGA with 5×5 array of logic blocks and 10-bit unidirectional buses. Most of the wire segments in between the core logic blocks and switch matrices are implemented in the CMOS metal layers. A total of 16 double-length wires use graphene, which interfaces to the switch matrices. Each segment has 4 redundant wires and a tester unit. The tester unit uses a coarse time-to-digital converter (TDC) to measure and convert the RC delay of the graphene wire into a 3-bit digital code. The TDCs have tunable resolution with a delay range between 1 ns and 20 µs.

1. X. Du, I. Skachko, A. Barker, and E. Y. Andrei, ”Approaching ballistic transport in suspended graphene,” Nature Nanotech., vol. 3, no. 8, pp. 491–495, 2008.
2. R. Murali, Y. Yang, K. Brenner, T. Beck, and J. D. Meindl, “Breakdown current density of graphene nanoribbons,” Appl. Phys. Lett., vol. 94, no. 24, p. 243114, 2009.
3. A. Naeemi and J. Meindl, “Conductance modeling for graphene nanoribbon (gnr) interconnects,” IEEE Electron Device Lett., vol. 28, no. 5, pp. 428–431, May 2007.
4. F. Li, Y. Lin, and L. He, “Vdd programmability to reduce FPGA interconnect power,” in Proc. IEEE/ACM International Conference on Computer Aided Design, 2004, pp. 760–765.
5. A. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M. S. Dresselhaus, and J. Kong, “Large area, few-layer graphene films on arbitrary substrates by chemical vapor deposition,” Nano Lett., vol. 9, no. 1, pp. 30–35, 2009.

Figure 1: (a-d) Transmission electron microscopy images of one to two layers of hexagonal boron nitride (hBN) film, (e) Selective electron diffraction pattern of hBN film, (f) Electron energy loss spectra of hBN film.

Hexagonal boron nitride (hBN) is very attractive for a variety of applications, particularly as deep ultraviolet emitter, transparent membrane, dielectric layer/substrate, or protective coating. In this work, we carried out detailed investigation of hBN synthesis on Cu substrate using chemical vapor deposition (CVD) with two heating zones under low pressure (LP).  Previously, few-layer hBN was synthesized via CVD under atmospheric pressure (AP) on metallic substrates ^{[1] [2]} .  In contrast, one- or two-layer hBN is synthesized under LPCVD. Ammonia borane, which is easier accessible and more stable in the atmosphere than borazine, is used as a BN precursor. These mono- or bi-layer hBN films are characterized by atomic force microscopy, transmission electron microscopy and electron energy loss spectroscopy analyses.  Our results suggest that the growth here occurs via surface-mediation, which is similar to graphene growth on Cu under low pressure. These atomically thin layers are particularly attractive for atomic membranes or dielectric layer/substrates for graphene devices.

1. Y. M. Shi, C. Hamsen, X. T. Jia, K. K. Kim, A. Reina, M. Hofmann, A. L. Hsu, K. Zhang, H. N. Li, Z. Y. Juang, M. S. Dresselhaus, L. J. Li, and J. Kong, “Synthesis of few-layer hexagonal boron nitride thin film by chemical vapor deposition,” Nano Letters, vol. 10, pp. 4134-4139, Oct. 2010.
2. L. Song, L. Ci, H. Lu, P. B. Sorokin, C. Jin, J. Ni, A. G. Kvashnin, D. G. Kvashnin, J. Lou, B. I. Yakobson, and P. M. Ajayan, “Large scale growth and characterization of atomic hexagonal boron nitride layers,” Nano Letters, vol. 10, pp. 3209-15, Aug. 2010.