Collaborators

• K. Aidala, Mt. Holyoke
• A. Akinwande, MIT
• N. Allam, Masdar Inst.
• M. A. Baldo, MIT
• M. Bawendi, MIT
• T. Bloomstein, Lincoln Lab.
• S. Gradečak, MIT
• F. Jaworski, Raytheon
• J. H. Lang, MIT
• M. A. Schmidt, MIT
• T. Swager, MIT

Postdoctoral Associates

• T. Andrew
• R. Costi
• Y. Gu
• K. Lo
• R. Lunt
• C. Packard
• K. Stone
• J. Summers
• W. Tisdale
• A. Wang
• E. Young

Graduate Students

• G. Akselrod, Physics
• P. Brown, Physics
• E. Flores, EECS
• A. Iacchetti, Polytech. Milan
• A. Murarka, EECS
• T. Osedach, Harvard Univ.
• S. Paydavosi, EECS
• S. Ramanan, EECS
• J. Rowehl, DMSE
• Y. Shirasaki, EECS
• G. Supran, DMSE
• M. E. Woo, DMSE

Support Staff

• M. Pegis, Admin. Asst.

SELECT Publications

(see onelab.mit.edu for full listing)

Barr, M.C., Rowehl, J.A., Lunt, R.R., Xu, J., Wang, A., Boyce, C.M., Im, S.G., V. Bulović, Gleason, K.K., “Direct monolithic integration of organic photovoltaic circuits on unmodified paper,” Advance Materials (2011).

Packard, C.E., Aidala, K.E., Samanan, S., V. Bulović, “Patterned removal of molecular organic films by diffusion,” Langmuir (2011).

Wood, V., Panzer, M.J., Bozyigit, D., Shirasaki, Y., Rousseau, I., Geyer, S., Bawendi, M.G., V. Bulović, “Electroluminescence from nanoscale materials via field-driven ionization,” Nano Letters (2011).

Brown, P.R., Lunt, R.R., Zhao, N., Osedach, T.P., Wanger, D.D., Chang, L-Y., Bawendi, M.G., V. Bulović, “Improved current extraction from ZnO/PbS quantum dot heterojunction photovoltaics using MoO₃ interfacial layer,” Nano Letters (2011).

Walker, B.J., Dorn, A., V. Bulović, Bawendi, M.G., “Color-selective photocurrent enhancement in coupled J-aggregate/nanowires formed in solution,” Nano Letters (2011).

Murarka, A., Packard, C., Yaul, F., Lang, J., V. Bulović, “Micro-contact printed MEMS,” IEEE 24th International Conference on Micro Electro Mechanical Systems, 292-295 (2011).

Young, E.R., Costi, R., Paydavosi, S., Nocera, D.G., V. Bulović, “Photo-assisted water oxidation with cobalt-based catalyst formed from thin-film cobalt metal on silicon photoanodes,” Energy & Environmental Science 4, 2058-2061 (2011).

Lunt, R.R., V. Bulović, “Transparent, near-infrared organic photovoltaic solar cells for window and energy-scavenging applications,” Applied Physics Letters 98, 113305 (2011).

Nausieda, I., Ryu, K.K., Da He, D., Akinwande, A.I., Bulović, V., Sodini, C.G., “Mixed-Signal Organic Integrated Circuits in a Fully Photolithographic Dual Threshold Voltage Technology,” IEEE Transactions on Electron Devices 58, 865-873 (2011).

Ren, S.Q., Zhao, N., Crawford, S.C., Tambe, M., Bulović, V., Gradečak, S., “Heterojunction Photovoltaics Using GaAs Nano-wires and Conjugated Polymers,” Nano Letters 11, 408-413 (2011).

Paydavosi, S., Abdu, H., Supran,G. J., V. Bulović, “Performance Comparison of Different Organic Molecular Floating Gate Memories,” IEEE Transactions on Nanotechnology 10, 594-599 (2011).

Osedach, T.P., Zhao, N., Geyer, S.M., Chang, L.Y., Wagner, D.D., Arango, A.C., Bawendi, M.G., V. Bulović, “Interfacial Recombination for Fast Operation of a Planar Organic/QD Infrared Photodetector,” Advanced Materials 22, 5250-5254 (2010).

Akselrod, G.M., Tischler, Y.R., Young, E.R., Nocera, D.G., V. Bulović, “Exciton-exciton annihilation in organic polariton microcavities,” Physical Review B 82, 113106 (2010).

Bradley, M.S., V. Bulović, “Intracavity optical pumping of J-aggregate microcavity exciton polaritons,” Physical Review B 82, 033305 (2010).

Panzer, M.J., Aidala, K.E., Anikeeva, P.O., Halpert, J.E., Bawendi, M.G., V. Bulović, “Nanoscale Morphology Revealed at the Interface Between Colloidal Quantum Dots and Organic Semiconductor Films,” Nano Letters 10, 2421-2426 (2010).

Zhao, N., T.P. Osedach, L.-Y. Chang, S.M. Geyer, D. Wanger, M.T. Binda, A.C. Arango, M.G. Bawendi, V. Bulović, “Colloidal PbS Quantum Dot Solar Cells with High Fill Factor,” ACS Nano 4, 3743-3752 (2010).

Optically transparent, wide band gap metal oxide semiconductors are a promising candidate for large area flexible electronics. Because most commercially available flexible substrates, particularly polymer substrates, cannot withstand the high temperature processing (>400°C) required for traditional silicon device fabrication, the development of new materials and devices that can be processed at low temperatures in a scalable manner is needed. Metal oxide semiconductors have been demonstrated to retain high carrier mobilities even in the disordered, amorphous state obtained when processed at near-room temperatures ^{[1] [2]} . Compared to amorphous silicon field effect transistors (FETs), which are the dominant technology used in display backplanes, metal-oxide-based FETs have been demonstrated with higher charge carrier mobilities, higher current densities, and faster response performance ^{[3] [4]} .

It has been shown both in simulation and by experiment that FET threshold voltage (V_T) can be modified simply by changing the channel layer thickness, without requiring the additional complexity of multiple channel materials or different dopings. In this project we have developed a low temperature (~100°C), scalable lithographic process for top-gate, bottom-contact amorphous metal oxide-based FETs using parylene, a room temperature-deposited CVD polymer, as gate dielectric. Figure 1 shows a micrograph of an array of FETs fabricated with different channel lengths. The baseline process was extended to enable the integration of FETs with different threshold voltages on the same substrate. The availability of FETs with different threshold voltages enables the implementation of enhancement/depletion (E/D) logic circuits that have faster speeds and smaller device areas than single-V_T topologies. Using the two-V_T lithographic process, we fabricated and characterized integrated E/D inverters and ring oscillators that operate rail-to-rail at supply voltages as low as V_DD = 3V. An example inverter characteristic is plotted in Figure 2. These results demonstrate the potential for low V_DD metal oxide-based integrated circuits fabricated in a low temperature budget, fully lithographic process for large area electronics.

1. K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors,” Nature, vol. 432, pp. 488-492, Nov. 2004.
2. J. Robertson, “Disorder and instability processes in amorphous conducting oxides,” Physica Status Solidi B-Basic Solid State Physics, vol. 245, pp. 1026-1032, June 2008.
3. R. L. Hoffman, B. J. Norris, and J. F. Wager, “ZnO-based transparent thin-film transistors,” Applied Physics Letters, vol. 82, pp. 733-735, Feb. 2003.
4. E. Fortunato, P. Barquinha, G. Goncalves, L. Pereira, and R. Martins, “High mobility and low threshold voltage transparent thin film transistors based on amorphous indium zinc oxide semiconductors,” Solid-State Electronics, vol. 52, pp. 443-448, Mar. 2008.

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.

Transformation of solar energy into chemical fuels is an attractive energy conversion transformation to address the intermittency of power generation in photovoltaics, which has been a long-standing challenge for development of solar energy ^[1] . Stored solar fuels may then be used when sunlight does not reach the solar panels, enabling continuous, solar-generated energy availability. The fuel cycle addressed in this work is the splitting of water to molecular hydrogen and oxygen, which can be used as fuels or as high-energy precursors in fuel synthesis.

We utilize a cobalt-containing water oxidation catalyst (Co-Pi) that assists the water to oxygen reaction ^[2] . Co-Pi, can be formed either via electrodeposition from Co²⁺ ions in aqueous solutions containing potassium phosphate (KPi) or by processing supported thin-film (800 nm thick) cobalt metal anodes in KPi at pH 7. Here, we use doped silicon wafers, Figure 1, as substrates for either electrodeposition of Co-Pi or processing of cobalt metal thin films into Co-Pi catalyst. Photoanode structures of ITO/Si/Co/Co-Pi, ITO/Si/ITO/Co-Pi and ITO/Si/ITO are prepared and tested for photo-assisted water oxidation activity.

Differences in water oxidation performance of each photoanode are particularly clear in Figure 2, which shows a comparison of steady-state current versus applied potential under dark and light conditions. The ITO/Si/ITO electrode passes very little current (10 μA/cm²) compared to the Co-Pi loaded electrodes and shows negligible change between light and dark conditions. The ITO/Si/ITO/Co-Pi electrode exhibits increasing dark current densities as voltage is applied and demonstrates additional increase under illumination (current offset between dark and light conditions reaches 200 μA/cm² at 1.35 V). The ITO/Si/Co/Co-Pi electrode achieves current densities higher than the ITO/Si/ITO/Co-Pi electrode in both dark and light conditions. The dark/light current offset over the catalytic regime of the electrode (between 0.85 V to 1.35 V) increases from 100 μA/cm2 (at 0.85 V) to 1 mA/cm² (at 1.35 V) and even higher. These results demonstrate the importance of material selection and device engineering for harnessing the solar spectrum towards catalytic chemical fuel production. The robust and abundant silicon electrode and direct processing of the cobalt metal produces an improved light-assisted water oxidation device.

1. T. R. Cook, D. K. Dogutan, S. Y. Reece, Y. Surendranath, T. S. Teets and D. G. Nocera, “Solar energy supply and storage for the legacy and nonlegacy worlds,” Chem. Rev. vol. 110, pp. 6474-6502, 2010.
2. M. W. Kanan and D. G. Nocera, “In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co²⁺,” Science vol. 321, pp. 1072-1075, 2008.

Figure 1: Schematic of the scanning near-field optical microscope utilizing FRET and a JCCR. (A) The sample is illuminated from the front of the JCCR through an objective lens. Fluorescence is collected through the same objective and a transparent non-interfering AFM probe tip is used. (B) Magnified cartoon of the tip-sample interaction in (A). The JCCR is frequency-matched to the absorption band of the analyte to be detected (a fluorescing small molecule, nanoparticles, or protein of interest). Light is absorbed in the J-aggregate thin film, and then its energy is transferred to the analyte. A fluorescence moiety is attached to the AFM probe tip. When the tip comes within the Förster radius of the analyte (typically 2-5 nm), fluorescence from the probe is detected in the far field.

The non-destructive chemical mapping of surfaces with single-molecule sensitivity and sub-5-nm lateral resolution is an extraordinary challenge. Optical techniques such as absorption, fluorescence, or Raman spectroscopy are powerful tools for chemical detection because of the unique optical signature that each molecule possesses. However, the maximum resolution of a classical (far-field) light microscope is limited by the fundamental law of diffraction. Typically, the resolving power of visible light cannot be better than ~200-300 nm in the lateral dimensions (x and y) and ~500-800 nm axially (z) ^[1] . Unfortunately, almost all biomolecules and chemical agents of interest are smaller than this resolution limit. To overcome these classical limitations, we are developing an ultrabright near-field fluorescence microscope capable of single-molecule detection sensitivity, chemical specificity, and sub-5-nm lateral resolution. Ultrahigh spatial resolution is achieved by utilizing a strongly distance-dependent near-field optical interaction called Förster resonance energy transfer (FRET). FRET is the near-field transfer of energy from a photoexcited fluorophore (the “donor”) to another nearby fluorophore (the “acceptor”). By attaching the acceptor species to a scanning probe tip, donor fluorophores can be spatially located on the sample surface within the Förster radius of the interaction (typically 2-5 nm, see Figure 1). Because single molecule fluorescence signals are weak, we employ an optical structure referred to as a J-aggregate critically coupled resonator (JCCR) to enhance the brightness of the donor ^[2] . The JCCR, which consists of a mirror, a λ/4 optical spacer, and a thin film (<15 nm thick) of J-aggregating dye molecules (see Figure 1), serves as a very general platform for efficient light capture and redistribution. Greater than 95% of incident light is absorbed within the thin J-aggregate film and then transferred to the donor species residing on the JCCR surface for subsequent FRET to the scanning probe tip.

1. L. Novotny, Principles of Nano-Optics. New York: Cambridge University Press, 2006.
2. J. R. Tischler, M. S. Bradley, and V. Bulović, “Critically coupled resonators in vertical geometry using a planar mirror and a 5 nm thick absorbing film” Optics Letters, vol. 31, pp. 2045-2047, July 2006.

Colloidal quantum-dot light-emitting diodes (QD-LEDs) combine the thin film processability of organic materials with the tunable optical properties conferred by QD size control. Archetypal QD-LED architectures have, to date, typically comprised a monolayer of QDs sandwiched between organic hole- and electron-transporting layers (HTL/ETL) (see Figure 1). Two possible QD excitation mechanisms have previously been proposed (Figure 1):  (red) direct carrier injection from organic transport layers into QDs, followed by exciton generation and recombination; and (green) Förster resonant energy transfer (FRET) of an exciton – initially generated in an organic transport layer – to the QD monolayer, followed by exciton recombination. Previous work ^[1] has demonstrated QD-LED systems in which the latter process appears to dominate. However, to date, all such architectures have comprised fluorescent small-molecule organics. Electrical excitation in the organic transport layer results in the creation of approximately one singlet exciton for every three triplet excitons ^[2] yet the latter typically go to waste because the lack of triplet oscillator strength in fluorophores prevents energy transfer. The possibility of harnessing these triplet states to as much as quadruple the efficiency of QD excitation motivates our ongoing efforts to use a phosphorescent sensitizer as a FRET donor for QDs. Analogous phosphor sensitization reported by Baldo et al ^[3] provides precedent for this work.

As a test bed for the study of phosphor sensitization, we have fabricated QD-LEDs (Figure 1) comprising monolayers of lead-sulfide (PbS) QDs (which yield near-infrared emission), obtained by micro-contact printing, previously reported by our group ^[4] . Devices with and without phosphor dopants in the hole-transport layer were compared. An increase in external quantum efficiency (EQE) upon introduction of the phosphor sensitizer was observed (Figure 2), though its extent (~ 20 to 50 %) was relatively slight, hinting at the possibility that direct charge injection may also play a role in the excitation of narrow band-gap QDs. To this end, we are currently investigating devices with wider band-gap QDs and employing steady-state and transient photoluminescence and photoluminescence excitation measurements to select complementary donor/acceptor systems.

1. P. O. Anikeeva, C. F. Madigan, J. E. Halpert, M. G. Bawendi, and V. Bulović, “Electronic and excitonic processes in light-emitting devices based on organic materials and colloidal quantum dots,” Physical Review B, vol. 78, pp. 085434-1 – 085424-8, Aug. 2008.
2. M. A. Baldo, D. F. O’Brien, M. E. Thompson, and S. R. Forrest, “The excitonic singlet-triplet ratio in a semiconducting organic thin film,” Phys. Rev. B, vol. 60, pp. 14422–14428, Nov. 1999.
3. M. A. Baldo, M. E. Thompson, and S. R. Forrest, “High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer,” Nature, vol. 403, pp. 750-753, Feb. 2000.
4. L. Kim, P. O. Anikeeva, S. A. Coe-Sullivan, J. S. Steckel, M. G. Bawendi, and V. Bulović, “Contact printing of quantum dot light-emitting devices,” Nano Letters, vol. 8, pp. 4513-4517, Oct. 2008.

J-aggregates, which are self-assembling molecular nanostructures ^[1] , have unique optical properties resulting from coherent coupling of their molecular components. For instance, J-aggregate films a few nanometers thick can strongly absorb light, making them interesting candidates for applications involving strong coupling in a microcavity ^[2] . In a J-aggregate, the peak absorption wavelength is red-shifted and narrowed with respect to the peak absorption wavelength of its molecular components due to dipole-dipole coupling, which produces a collective excitation, called an exciton, that is delocalized over the nanostructure ^[3] . Static and dynamic disorder in the aggregate affects the number of molecules over which the exciton is delocalized, therefore broadening the absorption line width. We use 2D Fourier transform optical spectroscopy to quantify the exciton size and determine its relationship to inhomogeneous (static) and homogeneous (dynamic) broadening.

Figure 1(a) shows a 2D correlation spectrum of a film of BIC ^[4] J-aggregates deposited on a sapphire substrate and cooled to 6 K in a cryostat. The negative-going (blue) feature indicates bleaching of the exciton transition while the positive-going (red) feature indicates excited state absorption to a two-exciton state. The energy separation of the two features is related to the exciton size ^[5] , similar to the model for a quantum-mechanical “particle-in-a-box.” Figure 1(b) shows the dependence of the exciton size on its absorption energy. Figure 1(c) shows that the average exciton size given by the pump-probe spectrum (17 molecules) does not reflect the value of the energy-dependent delocalization since the pump-probe peaks are broadened due to the inhomogeneity of the film.  As the temperature increases, the exciton size decreases, as Figure 2(a) shows, due to increased exciton-phonon scattering. The homogeneous line width also increases as the temperature increases, as in Figure 2(b), while the inhomogeneous line width remains constant, indicating that dynamic disorder influences the exciton size more strongly.

1. E. Jelley, “Spectral absorption and fluorescence of dyes in the molecular state,” Nature, vol. 138, pp. 1009-1010, Dec. 1936.
2. J. R. Tischler, M. S. Bradley, Q. Zhang, T. Atay, A. Nurmikko, and V. Bulovic, “Solid-state cavity QED: Strong coupling in organic thin films,” Organic Electronics, vol. 8, pp 94-113, Jan. 2007.
3. T. Kobayashi, ed., J-aggregates. Singapore: World Scientific, 1996.
4. The anionic J-aggregating molecule 5,6-dichloro-2-[3-[5,6-dichloro-1-ethyl-1,3-dihydro-3-(3-sulfopropyl)-2H-benzimidazol-2-ylidene]-1-propenyl]-1-ethyl-3(3-sulfopropyl)-1H-benzimidazolium, inner salt, sodium salt (CAS No. 28272-54-0) was dip-coated onto the substrate using a layer-by-layer deposition method along with the cationic polymer poly(diallyldimethylammonium chloride). The present sample had four bilayers of the cationic polymer and anionic dye.
5. M. van Burgel, D. A. Wiersma, and K. Duppen, “The dynamics of one-dimensional excitons in liquids,” Journal of Chemical Physics, vol. 102, pp. 20-34, Jan. 1995.

Colloidal quantum dots (QDs) are semiconductor nanoparticles that can be used in monolayers in low operating-voltage, non-lithographically fabricated, thin film LEDs exhibiting narrow-band emission ^[1] . Applications such as optical communication, spectroscopy, and sensing could benefit greatly from thin film devices that offer near-monochromatic, directed, and intense emission. The goal of our project is to demonstrate the feasibility of such devices by integrating a QD-LED into a planar resonant cavity (RC), akin to previous reports of RC structures ^{[2] [3]} , to enhance the intensity, directionality, and monochromaticity of the QD electroluminescence (EL).

The RC structure that we investigate (Figure 1) consists of a standard QD-LED ^[4] grown on top of a distributed Bragg reflector (DBR). An 8-pair DBR with an average reflectivity of 98 % between wavelengths of 575 nm and 625 nm is used to investigate the RC effect on a QD-LED. With this structure, we achieve a narrowed emission spectrum, which is evident when comparing spectra and photographs of the QD-LED and the RC QD-LED EL (Figure 2). The full width half maximum of the EL spectrum is reduced from 40 nm to 8 nm upon addition of the cavity.

To corroborate our experimental findings and to optimize the design of an RC QD-LED, we simulated the effect of the RC on the EL of the QD film. We first calculate the enhancement or suppression of the electric field at the location of the QD film for different wavelengths and different angles and then use Fermi’s golden rule to determine the transition rates for each of these conditions. As shown in Figure 2, the simulation is in good agreement with the experimental result, suggesting that the effect we observe is indeed an RC effect.

1. S. Coe, W. K. Woo, M. Bawendi, and V. Bulović, “Electroluminescence from single monolayers of nanocrystals in molecular organic devices,” Nature, vol. 420, pp. 800-803, Dec. 2002.
2. E. M. Purcell, “Spontaneous emission probabilities at radio frequencies,” Physical Review, vol. 69, pp. 681, Jun. 1946.
3. E. F. Schubert, N. E. J. Hunt, M. Micovic, R. J. Malik, D. L. Sivco, A. Y. Cho, and G. J. Zydzik, “Highly efficient light-emitting diodes with microcavities,” Science, vol. 265, pp. 943-945, Aug. 1994.
4. C. Wilmsen, H. Temkin, and L. Coldren, Vertical Cavity Surface Emitting Laser, Cambridge: Cambridge University Press, 1999.

As demands for high storage density, high chip memory capacity, and decreasing process costs continue to mount, conventional flash memory has found it challenging to continue scaling because of the minimum tunnel oxide thickness and poor charge retention due to defects in the tunneling oxide, necessitating modification in the implementation of the flash memory technology ^{[1] [2]} .

Molecular organic materials exhibit fascinating electronic properties that motivate their hybridization with traditional silicon-based memory devices in order to continue memory scaling. A floating gate consisting of a thin film of molecules would provide the advantage of a uniform set of identical nanostructured charge storage elements with high molecular area densities (e.g., 8 × 10¹³ cm^-2, which can result in a several-fold higher density of charge-storage sites as compared to QD memory and even SONOS devices) ^{[3] [4]} . Additionally, the discrete charge storage in such nano-segmented floating gate designs limits the impact of any tunnel oxide defects to the charge stored in the proximity of the defect site.

In order to study the memory behavior of organic molecules, we inject electrons/holes into the molecules by applying negative/positive bias to a conductive atomic force microscopy (AFM) tip (Pt probe tip with 10-nm radius) in contact with the organic layer (Figure 1). During the charge injection phase, the tip is brought into contact with the sample surface by reducing the amplitude setpoint to 0.5 V. The stored charges within molecules can be detected from surface potential mapping of the sample by Kelvin force microscopy (KFM). Figure 1 shows the KFM image of the charged spots by applying 9V tip bias. The minimal temporal decay of injected charges and their corresponding lateral spreading indicate highly localized charge distribution, suggesting potential use of small organic molecules in multi level trap based molecular flash memory cells with high storage capacity. The calculated stored charge density within molecules is shown in Figure 2.

1. P. Pavan, R. Bez, P. Olivo, and E. Zanoni, “Flash memory cells-An overview,” Proc. IEEE., vol. 85, no. 8, pp. 1248-1271,1997.
2. International Technology Roadmap for Semiconductors, ITRS. (2007). [Online]. Available: http://www.itrs.net.
3. P. K. Singh, R. Hofmann, K. K. Singh, N. Krishna, and S. Mahapatra “Performance and reliability of Au and Pt single-layer metal nanocrystal flash memory under NAND (FN/FN) operation,” IEEE Trans. Electron Devices, vol. 56, no. 9, Sep. 2009.
4. S. Paydavosi, H. Abdu, G. J. Supran, V. Bulović, “Performance Comparison of Different Organic Molecular Floating Gate Memories,” IEEE Trans. Nanotechnology, vol. 10, no. 3, May 2011.

It has been known for several decades that polymers doped with conducting particles, for example silicone nickel nano-particles, will exhibit a dramatically decreasing resistivity as the polymer is compressed. It is possible that the conductivity will vary by 12 orders of magnitude over a 40% strain ^[1] . Such composites conduct via tunneling from particle to particle, and the tunneling currents grow exponentially as the particles become closer together. These composites have already been used in applications from tactile sensors to fuses.

In this study we use the composites as the active element in an electronically-controlled switch. The Squeezable electronically controlled switch, referred to here as a “squitch,” is shown in Figure 1. In this embodiment, the squitch is a three-terminal device, with its terminals labeled as per the comparable terminals in a MOSFET. The central component of the device is the doped polymer labeled “Squitch Material” connected to drain and source electrodes. As fabricated, the squitch material would be a poor conductor, permitting little if any electron current to flow from the source to drain. That is, the resistance of this conduction path would be very large, putting the squitch in an off state. By applying voltage to the gate electrode, either positive or negative, an electric field is developed between the gate and the source. This electric field causes the gate to be attracted to the source, thereby compressing the squitch material. As the squitch material is compressed in the vertical direction, it begins to conduct, putting the squitch in an on state.

Thus, the squitch is a voltage controlled conductor, much the same as a FET or a BJT but with very large on-to-off conduction ratio and subthreshold swing (S) < 60 mV/dec, which allows for more aggressive supply voltage scaling and improvement in the energy efficiency.

1. D. Bloor, K. Donnelly, P. J. Hands, P. Laughlin, and D. Lussey, “A metal–polymer composite with unusual properties,” J. of Phys. D: Appl. Phys., 38 (2005) 2851–2860.