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Nanostructured silicon thin-film solar cells are promising, due to the strongly enhanced light trapping, high carrier collection efficiency, and potential low cost. Ordered nanostructure arrays, with large-area controllable spacing, orientation, and size, are critical for reliable light-trapping and high-efficiency solar cells. Available top–down lithography approaches to fabricate large-area ordered nanostructure arrays are challenging due to the requirement of both high lithography resolution and high throughput. Here, a novel ordered silicon nano-conical-frustum array structure, exhibiting an impressive absorbance of \({\sim }{99}\,\%\) (upper bound) over wavelengths 400–1100 nm by a thickness of only \(5\,\upmu \mathrm{{m}}\), is realized by our recently reported technique self-powered parallel electron lithography that has high throughput and high resolution. High-efficiency (up to 10.8 %) solar cells are demonstrated, using these ordered ultrathin silicon nano-conical-frustum arrays. Moreover, these ordered nano-structures have been successfully integrated into nano-electro-mechanical system (NEMS), enabling high-efficiency and broad-band optical actuation for NEMS devices. The first-ever nanopillar membrane acoustic speaker, using nano-scale photonic crystal optical absorbers for thermo-mechanical excitation of speaker membrane, is demonstrated.

The sensitivity of a micro/nano-scale mechanical mass sensor is limited by its frequency stabilities, which are affected by various frequency noises. Typically, device nonlinearity is intentionally avoided, because higher amplitude fluctuations in the nonlinear region could be translated into frequency variability. Here, we successfully used damping nonlinearity bifurcation to stabilize a mechanical membrane oscillator frequency to 0.04 ppm, a reduction by two orders of magnitude over that from linear motion. This method presents a general mechanism for oscillation frequency stabilization. We recently presented a DNA mass sensor with femto-molar sensitivity - coupled with the result in this paper, the sensitivity of the mass sensor could be improved by a factor of two orders of magnitude.

The widely used photodetector design based on atomically thin transition metal dichalcogenides (TMDs) has a lateral metal-TMD-metal junction with a fairly small, line shape photoresponsive active area at the TMD-electrode interface. Here, we report a highly efficient photodetector with extremely large photoresponsive active area based on a lateral junction of monolayer-bilayer WSe2. Impressively, the separation of the electron–hole pairs (excitons) extends onto the whole 1L–2L WSe2 junction surface. The responsivity of the WSe2 junction photodetector is over 3200 times higher than that of a monolayer WSe2 device and leads to a highest external quantum efficiency of 256% due to the efficient carrier extraction. Unlike the TMD p–n junctions modulated by dual gates or localized doping, which require complex fabrication procedures, our study establishes a simple, controllable, and scalable method to improve the photodetection performance by maximizing the active area for current generation.

Two-dimensional (2D) materials have emerged as promising candidates for miniaturized optoelectronic devices due to their strong inelastic interactions with light. On the other hand, a miniaturized optical system also requires strong elastic light–matter interactions to control the flow of light. Here we report that a single-layer molybdenum disulfide (MoS2) has a giant optical path length (OPL), around one order of magnitude larger than that from a single-layer of graphene. Using such giant OPL to engineer the phase front of optical beams we have demonstrated, to the best of our knowledge, the world’s thinnest optical lens consisting of a few layers of MoS2 less than 6.3 nm thick. By taking advantage of the giant elastic scattering efficiency in ultra-thin high-index 2D materials, we also demonstrated high-efficiency gratings based on a single- or few-layers of MoS2. The capability of manipulating the flow of light in 2D materials opens an exciting avenue towards unprecedented miniaturization of optical components and the integration of advanced optical functionalities. More importantly, the unique and large tunability of the refractive index by electric field in layered MoS2 will enable various applications in electrically tunable atomically thin optical components, such as micro-lenses with electrically tunable focal lengths, electrical tunable phase shifters with ultra-high accuracy, which cannot be realized by conventional bulk solids. Optical components that are just a few atomic layers thick have been made from the two-dimensional material molybdenum disulfide (MoS2). Researchers from the Australian National University and the University of Wisconsin in the USA fabricated a miniature concave optical lens that was just 6.3 nm thick by etching a several-layer-thick flake of MoS2 into a bowl shape. The lens had a focal length of –240 μm. In addition, the team fabricated highly efficient gratings by milling periodic patterns into MoS2 flakes up to six layers thick. Such ultrathin optical devices can be realized due to the very strong interaction between light and MoS2. In particular, the optical path length and elastic scattering efficiency of MoS2 are much larger than those of other materials.

Femtomolar concentration DNA detection is important as this is the concentration needed for early-stage cancer and bacterial infection diagnosis application [1]. Here, we present the first-ever nanomechanical mass-sensing resonator with ordered vertical nanowire (NW) arrays on top of a Si/SiO 2 bilayer thin membrane acting as a photonic crystal. The device has a very high surface area-to-volume ratio of 108 m−1, enabling DNA sensing of femtomolar concentration. Moreover, the nanowire array forms a photonic crystal that shows strong light trapping and absorption over broad-band optical wavelengths, enabling high-efficiency broad-band opto-thermo-mechanical remote device actuation and biosensing on chip. This method represents a mass-based platform technology that can sense molecules at low concentrations.

The challenge for new biosensors is to achieve detection of biomolecules at low concentrations, which is useful for early-stage disease detection. Nanomechanical biosensors are promising in medical diagnostic applications. For nanomechanical biosensing at low concentrations, a sufficient resonator device surface area is necessary for molecules to bind to. Here we present a low-concentration (500 aM sensitivity) DNA sensor, which uses a novel nanomechanical resonator with ordered vertical nanowire arrays on top of a Si/SiO2 bilayer thin membrane. The high sensitivity is achieved by the strongly enhanced total surface area-to-volume ratio of the resonator (108 m−1) and the state-of-the-art mass-per-area resolution (1.8×10−12 kg m−2). Moreover, the nanowire array forms a photonic crystal that shows strong light trapping and absorption over broad-band optical wavelengths, enabling high-efficiency broad-band opto-thermo-mechanical remote device actuation and biosensing on a chip. This method represents a mass-based platform technology that can sense molecules at low concentrations. Nanomechanical resonators are attractive as ultra-low concentration sensors of biomolecules, as their small scale allows for sensitive mass detection. Here, using a nanowire array as part of a photonic crystal, such a device is presented for light trapping, absorption and low-concentration sensing.

This paper presents the first nanopillar membrane acoustic speaker, using nanoscale photonic crystal optical absorbers for thermo-mechanical excitation of speaker membrane. The acoustic emission could be measured using B&K Sound Pressure Level Meter, corresponding to sound pressure levels of 50–60 dB. Optical actuation for MEMS takes advantage of directly coupling of energy into selected device areas without any electrical interconnects as required in integrated electrostatic and piezoelectric actuation. In opto-thermo actuation, light incident onto the structures is absorbed and converted to heat via photon absorption. Here, we realized linear, high efficiency and broad-band optical actuation for circular Si/SiO 2 membrane resonator with integrated vertical silicon nano-pillar photonic crystal arrays.

This paper presents the first-ever vertical silicon nanowire array based all-optical pressure sensor. Vertical silicon nanowire arrays with photonic crystal structure, can selectively trap or diffract light, depending on the optical wavelength, nanowire diameter, and nanowire pitch spacing. Here, we realize an all-optical pressure sensor by fabricating controllable vertical silicon nanowire arrays on a Si/SiO 2 membrane. Applying hydrostatic pressure bends the membrane, leading to membrane color change due to the modulation of the nanowire pitch and deflection angle. By acquiring the membrane image with a camera, the color image can be cross-correlated with a calibration data set to calculate pressure.

We used RadioIsotope Powered Electron Lithography (RIPEL), a new radioisotope emission-based electron lithography technique we recently presented [1], to fabricate a representative MEMS accelerometer. Because of nanoscale lithography dimensional control, the device shows high degree of dimensional matching (1–5%) between several devices. Moreover, measured accelerometer beam edge roughness is small (±10nm), which leads to the consistency between the measured pull-in-voltage and the theoretical calculation value. Furthermore, the small average deviation on the beam width (±15nm) leads to good pull-in-voltage uniformity from device to device (1–5%). Therefore, we demonstrate feasibility of using RIPEL to fabricate MEMS devices at nanoscale, with high precision, over large substrate for reduced cost.

Two-dimensional (2D) layered molybdenum disulfide (MoS2) has become a very promising candidate semiconducting material for future optoelectronic devices, owing to its unique properties. However, monolayer MoS2 is still a weak photon emitter, compared with other direct band gap semiconductors, which requires extra techniques or complicated steps to enhance its photon emission efficiency. Here, we demonstrated that nanostructured monolayer MoS2, produced by one-step chemical vapor deposition (CVD) growth, shows highly enhanced PL emission. The effective enhancement factor could be up to ∼43. Our results open the door to manipulating the optical properties of future devices by using nanostructured 2D monolayers.

The thin conformal HfO2 dielectric, which could provide very large geometric top-gate-capacitance Cgg comparable to SWNT quantum capacitance Cq, and the capacitance measurement technique developed by us, which could reduce the background capacitance down to C0 ~30aF, are two key promising factors for us to study the quantum capacitance of the SWNT FET. We successfully got the pronounced oscillating peaks in the Cq vs. top-gate VTG, which will be very usefully for us to better characterize the performance of the SWNT FETs and to further study the low-dimensional electronic structure.

Monolayer phosphorene provides a unique two-dimensional (2D) platform to investigate the fundamental dynamics of excitons and trions (charged excitons) in reduced dimensions. However, owing to its high instability, unambiguous identification of monolayer phosphorene has been elusive. Consequently, many important fundamental properties, such as exciton dynamics, remain underexplored. We report a rapid, noninvasive, and highly accurate approach based on optical interferometry to determine the layer number of phosphorene, and confirm the results with reliable photoluminescence measurements. Furthermore, we successfully probed the dynamics of excitons and trions in monolayer phosphorene by controlling the photo-carrier injection in a relatively low excitation power range. Based on our measured optical gap and the previously measured electronic energy gap, we determined the exciton binding energy to be ∼0.3 eV for the monolayer phosphorene on SiO2/Si substrate, which agrees well with theoretical predictions. A huge trion binding energy of ∼100 meV was first observed in monolayer phosphorene, which is around five times higher than that in transition metal dichalcogenide (TMD) monolayer semiconductor, such as MoS2. The carrier lifetime of exciton emission in monolayer phosphorene was measured to be ∼220 ps, which is comparable to those in other 2D TMD semiconductors. Our results open new avenues for exploring fundamental phenomena and novel optoelectronic applications using monolayer phosphorene. An optical scheme for determining the number of monolayers on two-dimensional materials has been developed. Research into two-dimensional materials is thriving, but developing a way to identify a single monolayer has proved challenging. Now, Jiong Yang and co-workers have used phase-shifting interferometry to deduce the number of phosphorene layers. They then performed power-dependent photoluminescence measurements to determine various excitonic properties of a monolayer on a silicon oxide/silicon substrate. They obtained an exciton binding energy of about 0.3 electron volts, which agrees well with theoretical predictions. The researchers measured a carrier lifetime of approximately 220 picoseconds, which is comparable to that of the transition-metal dichalcogenides, another class of two-dimensional semiconductors. They also measured a trion binding energy of about 100 milli-electron volts, which is around five times higher than that of transition-metal dichalcogenides.

Metallic and semiconducting carbon nanotubes generally coexist in as-grown materials. We present a gas-phase plasma hydrocarbonation reaction to selectively etch and gasify metallic nanotubes, retaining the semiconducting nanotubes in near-pristine form. With this process, 100% of purely semiconducting nanotubes were obtained and connected in parallel for high-current transistors. The diameter- and metallicity-dependent “dry” chemical etching approach is scalable and compatible with existing semiconductor processing for future integrated circuits.

We present a systematic experimental investigation of the reactions between hydrogen plasma and single-walled carbon nanotubes (SWNTs) at various temperatures. Microscopy, infrared (IR) and Raman spectroscopy and electrical transport measurements are carried out to investigate the properties of SWNTs after hydrogenation. Structural deformations, drastically reduced electrical conductance and increased semiconducting nature of SWNTs upon sidewall hydrogenation are observed. These changes are reversible upon thermal annealing at 500C via dehydrogenation. Harsh plasma or high temperature reactions lead to etching of nanotube likely via hydro-carbonation. Smaller SWNTs are markedly less stable against hydro-carbonation than larger tubes. The results are fundamental and may have implications to basic and practical applications including hydrogen storage, sensing, band-gap engineering for novel electronics and new methods of manipulation, functionalization and etching of nanotubes., Comment: 3 pages, 4 figures

For single-walled carbon nanotube (SWNT) field effect transistors, vertical scaling of high kappa dielectrics by atomic layer deposition (ALD) currently stands at approximately 8 nm with a subthreshold swing S approximately 70-90 mV/decade at room temperature. ALD on as-grown pristine SWNTs is incapable of producing a uniform and conformal dielectric layer due to the lack of functional groups on nanotubes and because nucleation of an oxide dielectric layer in the ALD process hinges upon covalent chemisorption on reactive groups on surfaces. Here, we show that by noncovalent functionalization of SWNTs with poly-T DNA molecules (dT40-DNA), one can impart functional groups of sufficient density and stability for uniform and conformal ALD of high kappa dielectrics on SWNTs with thickness down to 2-3 nm. This enables approaching the ultimate vertical scaling limit of nanotube FETs and reliably achieving S approximately 60 mV/decade at room temperature, and S approximately 50 mV/decade in the band-to-band tunneling regime of ambipolar transport. We have also carried out microscopy investigations to understand ALD processes on SWNTs with and without DNA functionalization.

This paper presents recent progress on placement and orientation control of single-walled carbon nanotubes (SWNTs) by both CVD and PECVD growth in electric fields and on single crystal quartz substrates, and post-growth Langmuir Bloddget assembly of close-packed SWNTs. We also present fabrication of nanotube field effect-transistors (FETs) including MOSFET like devices with 60m V/decade switching and ambipolar P-I-N band-to-band tunnel (BTBT) transistors with subthreshold swings down to 25m V/decade.

Electron beam exposure is the tool of choice for highest resolution lithography but suffers from the low throughput during serial beam writing [T. Ito and S. Okazaki, Nature (London) 406, 1027 (2000); R. F. Pease and S. Y. Chou, Proc. IEEE 96, 248 (2008)]. The authors designed and developed a low-cost self-powered near-field electron lithography (SPEL) technique, which utilizes the spontaneously emitted energetic electrons from beta-emitting radioisotope thin films. This approach enables massively parallel e-beam lithography, with potentially arbitrarily large concurrently exposed surface area, controlled by the size of the radioactive source. This method potentially eliminates the need for vacuum systems and the electron focusing column as needed in the existing electron beam lithography systems. This will greatly simplify the overall lithographic system and reduce the cost of deep-subnanometer lithography. In SPEL system, emitted electrons are spatially blocked using a nanostenciled micromachined mask t...

Electroluminescence of individual single-walled carbon nanotubes down to ∼15K is measured. We observe electrically driven light emission from suspended quasimetallic nanotubes in vacuum down to ∼15K and under different gas pressures at room temperature. Light emission is found to originate from hot electrons in the presence of electrically driven nonequilibrium optical phonons. Reduced light emission is observed in exponential manner as electron and optical phonon temperatures in the nanotube are lowered by lower ambient temperature or higher gas pressure. The results reveal over wide ambient conditions, light emission in a suspended tube is from thermally excited electron-hole recombination.