Figure 1: Triplet exciton transport in tetracene crystals with varying time delays. a, The prompt fluorescence profile. The image was taken with camera integration with a time delay of 0–0.5 μs upon laser excitation. b, c, d, e, Delayed fluorescence profiles with time delays of 0.5–1 μs, 1–2 μs, 2–3 μs, and 3 μs– upon laser excitations, respectively. f, Cross-section of images in a, b, c, d, e. The area of cross-section is denoted in image a.

Exciton transport is universal in every kind of organic optoelectronic devices – including organic light-emitting diodes (OLEDs) and organic solar cells ^[1] . In organic bilayer photovoltaic devices, exciton diffusion limits donor/acceptor thicknesses, restricting sufficient absorptions of photovoltaic materials. Exciton diffusion of singlet excitons (total spin of 0) is usually limited to tens of nanometers, significantly smaller than the absorption length at the visible spectrum (a ~ 1mm) ^[2] . However, triplet excitons (total spin of 1), having disallowed-transition to ground states, are capable of moving much longer distances, up to several mm ^[3] . The long-range triplet exciton transport can allow us to build more efficient solar cells.

Despite their long intrinsic lifetime, triplet diffusion is disorder-limited in amorphous or polycrystalline organic semiconductors ^[2] . Organic single crystals, however, provide defect-free environment where triplet excitons can diffuse over long distances without being quenched by defects ^[3] .

Long-range triplet exciton transport has been reported before in organic acene crystals. However, in previous studies, triplet exciton diffusion was measured using indirect methods, such as probing polarization- and wavelength-dependent photoconductivity ^[4] . In this work, we perform direct imaging of triplet excitons by monitoring delayed fluorescence in two archetypical organic single crystals: tetracene and rubrene crystals. The comparison between tetracene and rubrene crystals is interesting since they exhibit similar molecular structures but differ in crystal structures. Our study will contribute to a better understanding of long-range exciton transport and benefit the power conversion capability of organic solar cells by overcoming exciton diffusion bottlenecks.

1. 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, 2003.
2. R. R. Lunt, N. C. Giebink, A. A. Belak, J. B. Benziger, and S. R. Forrest, “Exciton diffusion lengths of organic semiconductor thin films measured by spectrally resolved photoluminescence quenching,” Journal of Applied Physics, vol. 105, pp. 053711, 2009.
3. M. Pope and C. E. Swenberg, Electronic Processes in Organic Crystals and Polymers. Oxford University Press, New York, 1999.
4. H. Najafov, B. Lee, Q. Zhou, L. C. Feldman, and V. Podzorov, “Observation of long-range exciton diffusion in highly ordered organic semiconductors,” Nature Materials, vol. 9, pp. 938, 2010.

Photolithography’s accuracy and scalability have made it the method for sub-micron-scale definition of single-crystal semiconductor devices for over half a century. Unfortunately, organic semiconductor devices are chemically incompatible with the types of resists, solvents, and etchants traditionally used. This work investigates the use of an uncommonly used chemically inert resist method ^[1] ((A. Han, D. Vlassarev, J. Wang, J. A. Golovchenko, and D. Branton, “Ice lithography for nanodevices,” Nano Letters, vol. 10, no. 12, pp. 5056-5059, Dec. 2010.))((D. Branton, J. A. Golovchenko, G. M. King, W. J. MoberlyChan, and G. M. Schürmann, “Lift-off patterning processing employing energetically-stimulated local removal of solid-condensed-gas layers” U.S. Patent 752443 B1, April 28, 2009.))((G. M. King, G. Schürmann, D. Branton, and J. A. Golovchenko, “Nanometer patterning with ice,” Nano Letters, vol. 5, no. 6, pp. 1157-1160, June 2005.))((J. Cuomo, C. Guarnieri, K. Saenger, and D. Yee, “Selective deposition with ‘dry’ vaporizable lift-off mask,” IBM Technical Disclosure Bulletin, vol. 35, no. 1, June 1992.)) that relies on physical phase changes for lift-off patterning of thin films of organic semiconductors and metals.

The resist gas is flowed over a cryogenically cooled substrate, where it freezes solid. This layer can be patterned by thermal excitation in a number of ways to define the areas where the desired thin film is to remain.  After the desired thin film or films are deposited, the substrate is brought up above the resist material’s sublimation point, leaving behind only the intended pattern. All the unwanted regions are lifted-off by the subliming resist.

Creating and defining the shadow mask on the surface of the substrate in this manner allow for patterning it with a stamp or roller with micron-scale features without changing the process conditions.  In this work, carbon dioxide is used as the sublimable mask material, and prototype stamps have been fabricated using SU-8 photoresist. A mask and the subsequent organic thin film are shown in Figure 2. This process may provide an alternative to shadow masks and provide a manufacturing solution for large area organic electronics.

1. W. Johnson, R. Laibowitz, and C. Tsuei, “Condensed gas, in situ lithography,” IBM Technical Disclosure Bulletin, vol. 20, no. 9, Feb. 1978.

Templated self-assembly of block copolymer, based on topographic templates defined by electron-beam lithography (EBL), is an attractive candidate for next generation high-resolution lithography. Templated self-assembly has two advantages compared with other lithography methods: first, the resolution can be scaled down to 5 nm, which cannot be achieved by optical lithography; second, the throughput can be increased by several folds compared with EBL. In our previous study, complex sub-20-nm patterns were fabricated with 45.5 kg/mol poly(styrene-block-dimethylsiloxane) (PS-b-PDMS) block copolymer ^[1] .

Here, we demonstrate high throughput sub-10-nm fabrication by using templated self-assembly of block copolymer. To achieve 10-nm resolution, the dimensions of a block copolymer and a topographic template were scaled down to 10-nm-length scale. We used 16 kg/mol PS-b-PDMS block copolymer, which yields 9-nm half-pitch PDMS cylinders. To control the orientation of 9-nm half-pitch PDMS cylinders, rectangular lattices of posts with height of 19 nm, diameter of 8 nm, and various periods were fabricated and annealed with the block copolymer. As a result, PDMS cylinders formed a long-range ordered region when the post array satisfied the commensurate condition. By varying the periods of posts, a broad range of block copolymer lattice orientation angles was achieved (Figure 1).

On a lattice with the period larger than 72 nm, PDMS cylinders lost long-range order. To further decrease the density of the posts and therefore increase the throughput without losing long-range order, a sparse lattice of dashes was tested. As a result, a region of well-aligned PDMS cylinders with width of 708 nm was achieved (Figure 2d). The dashes occupy only 1/66 of the final PDMS line pattern. This result suggests that if instead of writing the complete pattern, EBL is used to create template arrays and the pattern is then completed by a block copolymer, the throughput of EBL could be increased dramatically.

1. J. K. Yang, Y. S. Jung, J. Chang, R. A. Mickiewicz, A. Alexander-Katz, C. A. Ross, and K. K. Berggren, “Complex self-assembled patterns using sparse commensurate templates with locally varying motifs,” Nature Nanotechnology, vol. 5, pp. 256-260, Mar. 2010.

Semiconductor quantum dots (QDs) are electronically-quantized systems with promising applications in optoelectronic devices ^[1] . A key aspect of such systems is the fine control of optical transitions in the synthesis process ^[2] . These QDs are predominantly used in thin-film arrangement, deposited by spin casting or dip coating. Single QD patterning is one of the major challenges to designing a system that takes advantage of individual properties of QDs. Here we present a template self-assembly technique to control the position of individual QDs through electron-beam lithography (EBL). This optimized top-down lithographic process is a step towards the integration of individual QDs in optoelectronics systems for industrial applications.

The fabrication process of templated QDs is illustrated in Figure 1a. A poly(methylmethacrylate) (PMMA) resist was spin coated on a silicon substrate, followed by the fabrication of a mask through EBL. The size of the resulted PMMA templates was minimized by varying development temperature ^{[3] [4]} . Figure 1b shows the optimized PMMA patterning, with minimum template (i.e., hole) size of 8 nm for development at 6 °C. After defining the PMMA templates, a solution of QDs (6-nm-diameter CdSe) was spin casted and the remaining resist was removed by dissolution in acetone. This process resulted in QD clusters attached on the substrate. By optimizing the QD solution concentration, resist thickness, and feature size, we fabricated clusters with 1 to 10 QDs. One figure of merit in this process is the pattern yield, which is the ratio of yielded structures to the patterned templates. Figure 2 shows QD clusters with 87% pattern yield, with an average of 3 QDs in each cluster. Control of QD placement will be further optimized and integrated into photonic devices.

1. A. P. Alivisatos, “Semiconductor clusters, nanocrystals, and quantum dots,” Science, vol. 271. no. 5251, pp. 933-937, Feb. 1996.
2. S. A. Empedocles, D. J. Norris, and M. G. Bawendi, “Photoluminescence spectroscopy of single CdSe Nanocrystallite quantum dots,” Phys. Rev. Lett. vol. 77, pp. 3873-3876, Oct. 1996.
3. W. Hu, K. Sarveswaran, M. Lieberman, and G. H. Bernstein, “Sub-10 nm electron beam lithography using cold development of poly(methylmethacrylate),” J. Vac. Sci. Technol. B vol. 22, pp. 1711-1716, June 2004.
4. B. Cord, J. Lutkenhaus, and K. K. Berggren, “Optimal temperature for development of poly(methylmethacrylate),” J. Vac. Sci. Technol. B vol. 25, pp. 2013-2016, Dec. 2007.

Superconducting nanowire single-photon detectors (SNSPDs) ^[1] perform single-photon counting in the near‑infrared with outstanding performance. The main limitations of standard SNSPDs, based on ~ 4-nm-thick, 100-nm-wide NbN nanowires, are: (1) fragility with respect to constrictions; and (2) substantially reduced sensitivity beyond 2 µm wavelength (λ). We developed SNSPDs based on ultra-narrow (30- to 10-nm-wide) superconducting nanowires ^[2] , which showed improved robustness to constrictions and higher sensitivity to near-infrared photons with respect to standard SNSPDs.

As shown in Figure 1, at λ = 1550 nm our 30 nm nanowire‑width SNSPDs could be biased far from the device critical current (I_C) with minimal loss in detection efficiency (η), so even heavily‑constricted devices could reach the same efficiency as constriction‑free ones.

As shown in Figure 2 a, varying λ from 700 to 2100 nm, the η vs bias current (I_B) curves of 30 nm nanowire‑width SNSPDs kept a sigmoidal shape, with the cut-off current (I_co, taken to be at the inflection point of the η vs I_B curves) increasing from 0.31 I_C to 0.41 I_C. For 90 nm nanowire‑width detectors (shown in Figure 2 b), I_co increased from 0.64 I_C at λ = 500 nm to 0.89 I_C at λ = 1400 nm. This behavior indicates that ultra-narrow-nanowire SNSPDs are more sensitive to low-energy photons than standard devices and suggests that their sensitivity may extend to mid-infrared wavelengths.

The MIT Lincoln Laboratory portion was sponsored by the Department of the Air Force under Air Force Contract #FA8721-05-C-0002. Opinions, interpretations, recommendations and conclusions are those of the authors and are not necessarily endorsed by the United States Government.

1. G. N. Gol’tsman, O. Okunev, G. Chulkova, A. Lipatov, A. Semenov, K. Smirnov, B. Voronov, A. Dzardanov, C. Williams, and R. Sobolewski, “Picosecond superconducting single-photon optical detector,” Applied Physics Letters, vol. 79, no. 6, pp. 705-707, 2001.
2. F. Marsili, F. Najafi, E. Dauler, X. Hu, M. Csete, R. Molnar, and K. Berggren, “Single-photon detectors based on ultra-narrow superconducting nanowires,” Nano Letters, vol. 11, no. 9, pp. 2048-2053, 2011.

Figure 1: Scanning electron micrographs with uniform array of micropillars. The micropillars have the same height of 17 µm and pitch of 25 µm, while the diameters of pillars are (a) 6 µm, (b) 11 µm, and (c) 16 µm, respectively.

The mechanism of critical heat flux (CHF) is commonly attributed to two limits during boiling behavior: 1) the hydrodynamic limit due to Helmholtz instability and 2) the capillary limit determined by surface wettability ^[1] .  In recent years, a significant amount of research has been focused on CHF enhancement by utilizing micro/nanostructured surfaces to improve wettability ^{[2] [3] [4]} , with CHF of ~200 W/cm² being demonstrated ^[4] .  While most works are focused on making small structure sizes to improve surface wettability, the effect of this roughness-augmented wettability on CHF is poorly understood.  The limit of CHF enhancement with roughness-augmented wettability, where hydrodynamic instability becomes the dominant mechanism for CHF, has not been investigated.  In addition, boiling on nanostructured surfaces suffers from the requirement of high superheat due to bubble geometries closer to the homogeneous nucleation limit.  As a result, the heat transfer coefficient (HTC) on nanostructured surfaces is sacrificed ^[3] , which impairs the heat removal capability especially for applications demanding small temperature difference.

In this study, micro/nanopillar arrays are fabricated with a series of pitch and diameter size, as shown in Figure 1.  The sizes of pillar are designed to ensure that bubbles in the Cassie state, where vapor bubbles are suspended on the pillars, are energetically favorable such that bubble detachment is enhanced.  The series of sizes of the structured arrays generate various capillary forces, which allow the study on the mechanism for CHF and the limit of CHF enhancement with roughness-augmented wettability.  Furthermore, the investigation on surface roughness, where hydrodynamic instability dominants, gives the optimal size of structures for CHF enhancement and explores the feasibility of heterogeneous bubble nucleation on surfaces with proper structure geometry to reduce superheat.

1. S. G. Liter and M. Kaviany, “Pool-boiling CHF enhancement by modulated porous-layer coating: theory and experiment,” Int. J. Heat Mass Transfer, vol. 44, pp. 4287–4311, 2001.
2. C. Li and G. P. Peterson, “Parametric study of pool boiling on horizontal highly conductive microporous coated surfaces,” J. Heat Transfer, vol. 129, pp. 1465-1475, 2007.
3. S. Kim, H. D. Kim, H. Kim, H. S. Ahn, H. Jo, J. Kim, and M. H. Kim, “Effects of nano-fluid and surfaces with nano structure on the increase of CHF,” Exp. Therm Fluid Sci., vol. 34, pp. 487–495, 2010.
4. R. Chen, M.-C. Lu, V. Srinivasan, Z. Wang, H. H. Cho, and A. Majumdar, “Nanowires for enhanced boiling heat transfer,” Nano Lett., vol. 9, no. 2, pp. 548-553, 2009.

Figure 1: SEM, TEM, HR-TEM and SAED images of microsphere template InGaZnO3, illustrating a short range order of the spheres and amorphous phase of the sensor film.

Gas sensors are essential in the monitoring, control, and reduction of harmful emissions in the environment ^[1] .  Conductometric gas sensors based on semiconducting metal oxides are advantageous in many applications due to high sensitivity, manufacturability, and small size.  However, there are a number of drawbacks, including difficulty in control over the semiconductor/substrate interface, high power consumption, and reduced selectivity at high temperatures (300-400˚C) required for operation ^{[2] [3]} .  To address these challenges, chemical sensors comprising a wide array of material composition and morphology have been fabricated and investigated via high-throughput combinatorial test procedures.  A microsphere templating technique is employed in all device structures; it reduces the area of contact with underlying substrate and enhances interaction with the surrounding gases ^[4] .  Sensor performance has been characterized and optimized through controlled variation in the volume fraction of Pt nanoparticles that are co-deposited on the surface of SnO₂ and ZnO thin films.  In addition, novel sensors based on amorphous InGaZnO₄ have been investigated under a wide range of operating conditions and show promise for heightened sensitivity at reduced operating temperatures.  With a combination of rapid testing procedures and physical models of chemical and electronic processes involved in gas sensing, further advancements are anticipated in device sensitivity, selectivity, and response time.

1. F. Rock, N. Barsan, and U. Weimar ., “Electronic nose: Current status and future trends,” Chemical Reviews, vol. 108, no. 2, pp. 705-725, Jan. 2008.
2. K. J. Albert, N. S. Lewis, C.L. Schauer, G. A. Sotzing, S. E. Stitzel, T. P. Vaid, and D. R. Walt., “Cross-reactive chemical sensor arrays,” Chemical Reviews, vol. 100, no. 7, pp. 2595-2626, June 2000.
3. K. Wiesner, H. Knozinger,   M. Fleischer, H. Meixner, “Working mechanism of an ethanol filter for selective high-temperature methane gas sensors,” IEEE Sensors Journal, vol. 2, no. 4, pp. 354-359, Aug. 2002.
4. I. D. Kim, A. Rothschild, T.Hyodo, and H. L. Tuller,, “Microsphere templating as means of enhancing surface activity and gas sensitivity of CaCu₃Ti₄O₁₂ thin films,” Nano Letters, vol. 6, no. 2, pp. 193-198, Jan. 2006.

Detection of high-energy radiation (e.g., γ-rays) is key in nuclear non-proliferation strategies.  When a wide-band gap semiconductor detector intercepts a γ-ray, electron–hole pairs are formed, resulting in an increase in electrical conductivity. This change in conductivity, or sensitivity, is maximized if the conductivity in the non-illuminated (dark) state is very low.  In order to achieve high sensitivity, current semiconductor technologies require device cooling to very low temperature, which adds to cost and reduces portability.  TlBr is an attractive detector material because of its low dark conductivity at room temperature as well as its high mass density, leading to higher radiation absorption.

In this project, we have characterized the dominant ionic conduction properties in TlBr using impedance spectroscopy.  Through doping techniques, we have determined that TlBr is primarily a Schottky-type ionic conductor, meaning that Tl and Br move through the material by ionic vacancy motion.  These measurements have led us to predict a doping strategy to minimize the dark conductivity, and we are collaborating with a local company (RMD) to implement this technology as well as developing it further by studying new TlBr-based material systems.  In addition, our newfound understanding of TlBr has led us to investigate novel device designs never before used in ionic conducting systems.

Combustion of fossil fuels, essential for electricity generation and vehicular propulsion, is generally incomplete, leading to harmful NOx, CO, and unburned hydrocarbons emissions.  Great progress in minimizing such emissions has relied on the operation of “three-way catalysts” (TWCs), which utilize a combination of precious metals and metal oxides with the ability to take up or release oxygen for reduction/oxidation of pollutants (NOx to N₂ plus CO and HC to CO₂ and H₂O, respectively).  In this project, we are investigating the rate at which oxygen storage materials (OSM) exchange oxygen with the atmosphere and the magnitude of oxygen they store with the aid of geometrically well-defined thin film structures. Impedance spectroscopy, Kelvin probe, and thermogravimetric methods are used to determine electrochemical performance and oxygen storage capabilities.  These properties, when correlated to actual TWC performance using a differential flow reactor, will allow for a more detailed understanding of performance criteria.

Vapor deposited thin films are rarely stable, so that when they are heated to temperatures at which atomic diffusivities are sufficiently high, they will dewet to form isolated islands. This liquid-like process, driven by surface energy minimization, can occur well below the film’s melting temperature, so that structure evolution occurs via surface self-diffusion on the solid film.  Film dewetting (sometimes called agglomeration) has long been a problem in the processing of micro-systems.  Dewetting of silicides, metal films, and even silicon-on-insulator films has been a concern that required careful process control to avoid.  At the same time, dewetting has also come to be appreciated as a means of producing catalysts for nanowire and nanotube growth and, increasingly, arrays of more complex structures.

In the past, we have studied dewetting of polycrystalline to understand the conditions required to avoid dewetting, as well as to develop techniques for control of dewetting to give ordered arrays of catalysts.  In recent work, we have studied dewetting of single crystal films, using epitaxial Ni films on MgO as a model system.  We found that pre-patterning of the films can be used to reproducibly guide the formation of complex structures.  Pattern formation is strongly affected by the size of the patterning relative to the film thickness, and by crystalline anisotropy of the surface energy.  Shape evolution is governed by a specific set of fundamental processes that include formation and pinch-off of rims to form wires ^[1] , corner and edge instabilities ^[2] , edge faceting ^[3] , and Rayleigh-like instabilities to form islands from lines ^[4] .  Relatively simple pre-patterning can be used to reproducibly form patterns with more complex shapes and smaller feature sizes.

1. J. Ye and C. V. Thompson, “Regular pattern formation through the retraction and pinch-off of edges during solid-state dewetting of patterned single crystal films,” Physics Review, vol. B82, p. 193408, 2010.
2. J. Ye and C. V. Thompson, “Mechanisms of complex morphological evolution during solid-state dewetting of single-crystal nickel thin films,” Appl. Phys. Letts., vol. 97, p. 071904, 2010.
3. J. Ye and C. V. Thompson, “Anisotropic edge retraction and hole growth during solid-state dewetting of single crystal nickel thin films,” Acta Materialia, vol. 59, p. 582, 2011.
4. J. Ye and C. V. Thompson, “Templated solid-state dewetting to controllably produce complex patterns,” Advanced Materials, vol. 23, p. 1567, 2011.