Visible Light Photodetector Research Articles

2D MoS2/CuPc heterojunction based highly sensitive photodetectors through ultrafast charge transfer

Photodetectors (PD) with high detectivity, high quantum efficiency and fast response are fundamental units to achieve sensors, imagers, and many others functional optoelectronics. Here we demonstrate high performance vertical stacked 2D MoS2/CuPc heterojunction based photodetectors with high sensitivity, high external quantum efficiency and ultrafast response time. The functional p-type organic thin film of CuPc was thermally evaporated on n-type two-dimension (2D) MoS2 monolayer to create an ideal type II heterojunction interface, where ultrafast charge transfer (CT) occur in 16 ps, followed by effective exciton separation and charge collection. As a result, photoresponse time as fast as 436 μs has been obtained for 2D MoS2/CuPc PDs, as well as a profound responsivity of 3.0×103A/W and a detectivity of 2.0 × 1010 Jones with a peak external quantum efficiency (EQE) of 483%. This work suggests a feasible route to develop ultrasensitive visible light photodetector by 2D materials.

Rhenium substitutional doping for enhanced photoresponse of n-SnSe2/p-Si heterojunction based tunable and high-performance visible-light photodetector

The low photoresponse of solution-processed 2D/3D heterojunctions is still a challenging factor in terms of the efficient output of the device as a photodetector. Here, we have doped tin diselenide (SnSe2) with Rhenium (Re) transition metal by Direct Vapor Technique (DVT). The bulk doped SnSe2 then scale down to the nanoscale dimension by liquid-phase exfoliation (LPE) method. Synthesized nanocrystals exhibit the multilayered structure and nanometer dimensions with a good crystalline structure. The incorporation of Re is confirmed by EDAX, XRD, and Raman Spectroscopy. The Re incorporation in SnSe2 host lattice enhances the absorption in the visible region as compare to pristine SnSe2. The n-type Re doping decreases the turn-on voltage extracted from the Si-based heterojunction device measurements. The doped n-SnSe2/p-Si photodetector achieved the peak photoresponsivity (Rλ) and Detectivity (D*) of 7.90 A/W and 16.47 × 1012 respectively, at −2 V, which are ~2.8 and ~2.6 times higher than that of the pristine n-SnSe2/p-Si photodetector. This can be attributed to pronounce light-matter interaction and shifting of Fermi level towards to conduction band providing more charge carriers to participate in electrical conduction. Our work provides archetype for metal doping in multilayered semiconductor for high-performance optoelectronics devices.

Mesoporous ZnO/ZnAl2O4 mixed metal oxide-based Zn/Al layered double hydroxide as an effective anode material for visible light photodetector

In this article, mesoporous ZnO/ZnAl2O4 mixed metal oxide (MMO) based Zn/Al layered double hydroxide (LDH) was prepared via modified co-precipitation and thermal reduction methods for visible light photodetector. A series of photodetectors were assembled using the corresponding mesoporous MMO attended with Ruthenizer dye as visible light sensitizer (535-bisTBA). Particularly, the amount of dye loaded onto the mesoporous layers was estimated through UV–Vis analysis-based regression equation, which provides a solid correlation with the fabricated device's photo-responsive performance. Simultaneously, a considerable photo-responsivity and specific photo-detectivity of the fabricated device was perceived. This could be attributed to the significant BET surface area (53 m2/g) as well as high loaded amount of dye on the prepared mesoporous layer (0.33 mMol/cm−2). In detail, the photo-responsivity and specific photo-detectivity for ZA-400 photodetector were found to be within the range of 11 mA/mW and 1011 jones, respectively; in which a superior photo-responsive behavior was acquired as compared to similar geometries. The optimum device (ZA-400) exhibited an approximate response time of 900 m s. In addition to the consistency and robustness of the proposed photodetector, this work offers an alternative approach for eco-friendly optoelectronic design.

Enhancement in photodetection properties of PbI2 with graphene oxide doping for visible-light photodetectors

Herein, lead iodide (PbI2) nanorods/graphene oxide (GO) nanocomposites have been successfully achieved at low temperature via microwave-assisted syntheses route. The X-ray diffraction (XRD) analysis confirm good crystallinity and absence of any impurity in the prepared hybrid nanostructures which further confirmed through Raman analysis. The XRD and Raman study also confirm the doping of GO in 2H polytypic PbI2 structure. The average crystallite size of PbI2 is calculated to be 20 nm which found to be increased to 53 nm in the case of GO-PbI2 for high intensity peak. Scanning electron microscope (SEM) images reveal that the GO sheets are decorated with PbI2 nanoparticles and nanorods. The absorption spectra of PbI2 was found to broaden whereas energy bandgap of GO-PbI2 decreases to 2.83 eV in compare to 3.24 eV for PbI2. The effect of graphene oxide doping on photodetector performance of PbI2 was elucidated by measuring the I-V characteristics under 532 nm laser light illumination. The pure PbI2 exhibits a stable and reversible photodetector performance with responsivity of 0.060 AW−1, detectivity of 6.38 × 1010 Jones, EQE of ∼14 % and response time of 224 ms. For GO-doped PbI2, these parameters were found to be improved to 0.182 AW−1, 1.79 × 1011 Jones, ∼42 %, and 128 ms, respectively.

Topotactic transition of Pb0.99Bi0.01I2 into CH3NH3Pb0.99Bi0.01I3 on TiO2 for high-performance visible light perovskite photodetector

In this article, we report the fabrication of self-powered CH3NH3Pb0.99Bi0.01I3 perovskite visible light photodetector by chemical vapour deposition. The crystalline Pb0.99Bi0.01I2 layer grown on TiO2 nanoarrays (NAs) was transformed into trap free CH3NH3Pb0.99Bi0.01I3 functional perovskite layer by exposure of methylamine iodide at various time intervals. The TiO2 NAs improve the morphology and growth of crystalline CH3NH3Pb0.99Bi0.01I3 layers which increases the charge transport and exciton dissociation. Self-powered device resulted in the high photocurrent density of 9.01 × 10−4, 2.65 × 10−3 and 7.06 × 10−3 mA/cm2 under the irradiation of simulated visible solar light at various power densities of 30, 60 and 90 mW/cm2, respectively. The dark current of 10−4 mA/cm2 at 0.3 V clearly evidences ultra-low charge carrier recombination in the bilayer photodetector device. The device accomplishes very good photoresponsivity of ~10−4 AW−1 under self-powered condition.

Microplasma-Enabled Graphene Quantum Dot-Wrapped Gold Nanoparticles with Synergistic Enhancement for Broad Band Photodetection.

Plasmonic nanostructure/semiconductor nanohybrids offer many opportunities for emerging electronic and optoelectronic device applications because of their unique geometries in the nanometer scale and material properties. However, the development of a simple and scalable synthesis of plasmonic nanostructure/semiconductor nanohybrids is still lacking. Here, we report a direct synthesis of colloidal gold nanoparticle/graphene quantum dot (Au@GQD) nanohybrids under ambient conditions using microplasmas and their application as photoabsorbers for broad band photodetectors (PDs). Due to the unique AuNP core and graphene shell nanostructures in the synthesized Au@GQD nanohybrids, the plasmonic absorption of the AuNP core extends the usable spectral range of the photodetectors. It is demonstrated that the Au@GQD-based visible light photodetector simultaneously possesses an extraordinary photoresponsivity of ∼103 A/W, ultrahigh detectivity of 1013 Jones, and fast response time in the millisecond scale (65 ms rise time and 53 ms fall time). We suggest that the synergistic effect can be attributed to the strong fluorescence quenching in Au@GQD coupled with the two-dimensional graphene layer in the device. This work provides knowledge of tailoring the optical absorption in GQDs with plasmonic AuNPs and the corresponding photophysics for broad band response in PD-related devices.

Development and characterization of TlGaSe2 thin film-based photodetector for visible-light photodetector applications

In current work a thermal evaporation technique has been employed to fabricate the thin films from the grown TlGaSe2 single crystal and investigated. X-ray diffraction study revealed the formation of polycrystalline films of monoclinic crystal system. The crystallite size was estimated to be 11 nm. Atomic force and scanning electron microscopy studies shows the nanocrystalline film fabrication and the size of grain was estimated. Optical study shows that the grown film is about 55% transparent in 800–1500 nm region and possess a sharp absorption edge. The direct energy gap of TlGaSe2 films was estimated around 2.31 eV. The photo current of the fabricated TlGaSe2 photodetector increases about 6 times compare to dark when exposed under 5 mW/cm2 illumination. The fabricated detector possesses high external quantum efficiency of 158% and also the detectivity reached to 5.16 × 1010 at V = 10 V. The on/off behaviour of the device was also studied and found that the response time for growth and decay is 88 ms and 90 ms, respectively.

Non-Contact Vital Signs Monitoring Through Visible Light Sensing

This paper presents a non-contact vital signs (respiration and heartbeat) monitoring system that utilizes visible light sensing (VLS) technology. We have for the first time demonstrated the ability to wirelessly (non-contact) sense vital signs using only reflected incoherent light signals from a human subject. The VLS-based system is implemented by using simple visible light source, photodetector and data acquisition/processing unit, and is used with the developed signal processing algorithms to turn slight variations in reflected light power into accurate measurements of respiration and heart rate. To assess the accuracy of our method, the results were compared with reliable measurements using a contact-based monitoring device (ground truth). More than 94% of accuracy was observed in test results including both breathing and heartbeat rates in different scenarios as compared to the state-of-the-art baseline methods such as contact-based vitals monitoring devices. These competitive results have demonstrated that VLS-based vitals monitoring innovation is indeed a viable, powerful, attractive, low-cost and safe method. This study represents a substantive departure from the traditional ways of doing non-contact vitals monitoring methods (e.g., radio-frequency-based radars and imaging-based cameras) and is poised to make big contributions to this area. Since vital signs monitoring is a ubiquitous element of medicine, this work would also impact the entire health care community, from patients in their homes, to doctor’s offices, to large medical institutions and industries. This technology has potential to address numerous conditions and situations in which vital signs are critical indicators such as sleep apnea and human-computer-interaction applications.

Impact of CsI concentration, relative humidity, and annealing temperature on lead-free Cs2SnI6 perovskites: Toward visible light photodetectors application

Physical, electrical, and photodetection properties of stable, lead-free Cs2SnI6 (CSI) perovskite thin films prepared on thermally oxidized Si (SiO2, 100 nm) substrates via direct solution spin coating process are reported. XRD, XPS, and EDS analysis reveal that initial CsI rich-concentration, higher RH% levels (>40%) have an impact on phase formation of CSI films. The porous, sub-μm sized rods of fabricated CSI films are confirmed by FEG-SEM analysis. Contact resistance (Rc) and sheet resistance (Rsh) for CSI films, annealed at 150 °C, are decreased up to 37% and 85%, respectively, as compared with those of films processed at 75 °C. The photoresponsivity (6 mA/W) and specific detectivity (2.00 × 109 Jones) for two-terminal CSI photodetectors are achieved with annealing condition of 100℃ and at a bias voltage of 1 V as well. The preliminary photodetection properties for CSI devices processed at low temperatures indicate that they can be potentially applicable to a flexible, visible light photodetector.

Ultraviolet Photodetecting and Plasmon-to-Electric Conversion of Controlled Inkjet-Printing Thin-Film Transistors

Direct ink-jet printing of a zinc-oxide-based thin-film transistor (ZnO-based TFT) with a three-dimensional (3-D) channel structure was demonstrated for ultraviolet light (UV) and visible light photodetection. Here, we demonstrated the channel structures by which temperature-induced Marangoni flow can be used to narrow the channel width from 318.9 ± 44.1 μm to 180.1 ± 13.9 μm via a temperature gradient. Furthermore, a simple and efficient oxygen plasma treatment was used to enhance the electrical characteristics of switching ION/IOFF ratio of approximately 105. Therefore, the stable and excellent gate bias-controlled photo-transistors were fabricated and characterized in detail for ultraviolet (UV) and visible light sensing. The photodetector exhibited a superior photoresponse with a significant increase of more than 2 orders of magnitude larger drain current generated upon UV illumination. The results could be useful for the development of UV photodetectors by the direct-patterning ink-jet printing technique. Additionally, we also have successfully demonstrated that a metal-semiconductor junction structure that enables plasmon energy detection by using the plasmonic effects is an efficient conversion of plasmon energy to an electrical signal. The device showed a significant variations negative shift of threshold voltage under different light power density with exposure of visible light. With the ZnO-based TFTs, only ultraviolet light detection extends to the visible light wavelength.

Transferrable thin film of ultrasonically exfoliated MoSe2 nanocrystals for efficient visible-light photodetector

Abstract Two-dimensional transition metal dichalcogenides (2D TMDC's) materials are the impactful candidates in nanotechnology for diverse applications such as electrocatalyst, sensing and energy storage. Many efforts have been accomplished to solve the problem of green production with the scaling of TMDCs materials. Here we demonstrated the two-step method including electrophoresis deposition (EPD) and direct transfer of MoSe2 nanocrystals (NCs) thin film for fabricating visible-light photodetector. High-resolution transmission electron microscopy and atomic force microscopy show that the liquid exfoliated MoSe2 NCs exhibits good crystalline nature with multi-layered thickness. Raman spectra, UV–Visible spectra, and X-ray photoelectron spectroscopy confirm the presence of 2H-phase of MoSe2 NCs. EPD deposited thin film is detached by dipping into the water and transferred onto the etched ITO substrate. Ag/MoSe2 NCs/ITO photodetector is fabricated which demonstrated the excellent performance in the visible range and demonstrate remarkable values of responsivity, EQE (%) and specific detectivity of 566 mAW−1, 155% and 3.69 × 1011 Jones. Considering our results, the fabrication technique along with the photoresponse application of TMDC NCs presented here is the innovative and efficient way for developing the large-area optoelectronic devices.

Perovskite-Inspired Lead-Free Ag2BiI5 for Self-Powered NIR-Blind Visible Light Photodetection

HighlightsThe photodetection capabilities of emerging perovskite-inspired lead-free Ag2BiI5 are investigated.In self-powered mode, a near-constant photoresponse through the visible with a NIR rejection ratio of > 250 is obtained.Optoelectronic characterization provides insight into the interplay among efficiency, collection distance, and film micro-/nano-structure.

Investigations on the effects of rGO incorporation on the photosensitivity of (Cd:Zn)S nanocrystalline thin film-based visible photodetectors by hydrothermal synthesis

Reduced graphene oxide-incorporated cadmium zinc sulphide (rGO–CZS) thin films were synthesised on glass substrates by hydrothermal-assisted chemical bath deposition. The effect of the rGO concentration (0.5–2 wt%) on the photosensitivity of the films was established along with the structural, morphological and optical properties of the composite films. The structural investigations by XRD analysis ensured that the prepared samples are nanocrystalline with hexagonal structure of cadmium zinc sulphide. Scanning electron microscopy showed aggregated sphere-like CZS nanoparticles enveloped with rGO flakes. The average particle size attained from TEM image is approximately 8 nm. The presence of various functional groups was scanned by FTIR analysis. Further, we established the characteristics of the rGO–CZS nanocomposite films as visible-light photodetectors. The photosensitivity, photo response and detectivity of the devices were found to increase with rGO concentration. (2 wt%) rGO–CZS detector exhibits highest photosensitivity of the order of 106, photoresponse of 18.5 mA/W and specific detectivity of 2.08 × 1012 Jones. This work demonstrates fast and simple preparation process of rGO–CZS visible-light detectors with high sensitivity, responsivity and good stability. The enhancement of sensitivity and responsivity can be attributed to the excellent electron conductivity and mobility due to the warm interfacial contact between CZS nanoparticles and the rGO sheet.

Field effect transistor photodetector based on two dimensional SnSe<sub>2</sub>

Two dimensional materials have been attracting intensive interest due to their unique physical and optoelectronic properties. As an emerging two dimensional materials, SnSe<sub>2</sub> have shown a considerable potential for next-generation electronic and optoelectronic. Herein, SnSe<sub>2</sub> bulk crystals have been prepared by a chemical vapour transport method with high purity tin and selenium powder as precursors. Then SnSe<sub>2</sub> multilayers has been successfully prepared by a micromechanical exfoliation method from the SnSe<sub>2</sub> bulk crystals. The phase structures and elemental composition of the bulk crystal are investigated using an X-Ray diffractometer, an X-ray photoelectrons spectrometer and a Raman spectrometer. And the morphologies are observed using an optical microscope, an atomic force microscope and a transmission electron microscope. The measurement results show that the SnSe<sub>2</sub> bulks are single crystals with a high crystallization and purity. The SnSe<sub>2</sub> multilayers have a size of 25–35 μm and a thickness of 1.4 nm. To detect the electronic and photoresponse characteristics of the SnSe<sub>2</sub> multilayers, a field effect transistor based on such SnSe<sub>2</sub> are fabricated via a photolithographic-pattern-transfer method. The transistor has a smooth surface without wrinkles and bubbles, and also has a good contact with Au electrodes. The transistor shows a linear output characteristic and an obvious rectification. The on/off ratio of the device is 47.9 and the electron mobility is 0.25 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>. As a photodetector, the field effect transistor exhibits obvious photoresponse to three visible lights with the wavelengths of 405, 532, and 650 nm. As the lasers are turned on and the device is under illuminations of three visible lights, the current increase rapidly to a saturation state. Then as the lasers are switched off, the current decrease and recover to the original state. The drain-source current can alternate between high and low states rapidly and reversibly, which demonstrates photoresponse characteristics of the devices are stable and sensible. Notably, it shows a strongest response to the 405 nm light at an intensity of 5.4 mW/cm<sup>2</sup> with a high responsivity of 19.83 A/W, a good external quantum efficiency of 6.07 × 10<sup>3</sup>%, a normalized detectivity of 4.23 × 10<sup>10</sup> Jones, and a fast response time of 23.8 ms. The results of this work demonstrate that layered SnSe<sub>2</sub> can be a suitable and excellent candidate for visible light photodetector and has a huge potential for high-performance optoelectronic devices.

Edge-Epitaxial Growth of InSe Nanowires toward High-Performance Photodetectors.

Semiconducting nanowires offer many opportunities for electronic and optoelectronic device applications due to their unique geometries and physical properties. However, it is challenging to synthesize semiconducting nanowires directly on a SiO2 /Si substrate due to lattice mismatch. Here, a catalysis-free approach is developed to achieve direct synthesis of long and straight InSe nanowires on SiO2 /Si substrates through edge-homoepitaxial growth. Parallel InSe nanowires are achieved further on SiO2 /Si substrates through controlling growth conditions. The underlying growth mechanism is attributed to a selenium self-driven vapor-liquid-solid process, which is distinct from the conventional metal-catalytic vapor-liquid-solid method widely used for growing Si and III-V nanowires. Furthermore, it is demonstrated that the as-grown InSe nanowire-based visible light photodetector simultaneously possesses an extraordinary photoresponsivity of 271 A W-1 , ultrahigh detectivity of 1.57 × 1014 Jones, and a fast response speed of microsecond scale. The excellent performance of the photodetector indicates that as-grown InSe nanowires are promising in future optoelectronic applications. More importantly, the proposed edge-homoepitaxial approach may open up a novel avenue for direct synthesis of semiconducting nanowire arrays on SiO2 /Si substrates.

Double perovskite Cs4CuSb2Cl12 microcrystalline device for cost effective photodetector applications

Recently reported double perovskites (DPs), Cs4CuSb2Cl12 has led to the development of a new lead free perovskite. It is 1 eV band gap material. The perovskite microcrystals were synthesized via liquid phase method. This was investigated as active solar energy materials for visible-light photodetector. The microcrystalline device was made in a single-step printable Cs4CuSb2Cl12/carbon materials composite paste. 10 wt% active material composite was found to be the best candidate for the highest responsivity, 10-3 A/W and detectivity, 108 J. One-step printing of the composite paste can be used for cost effective photodetector applications.

High performance graphene-like thinly layered graphite based visible light photodetector

In this work, the drop casting method is utilized to obtain a thin graphene-like graphite layer as a photodetection layer on a SiO2/Si substrate, and its performance is characterized to determine its potential use in commercial applications. The thin layer is formed by first synthesizing graphite soot from a butane flame before being turned into a liquid solution and drop-casted onto the substrate to form a thin graphene-like graphite film. Field emission scanning electron microscope and Raman vibrational mode analyses indicate that the fabricated graphene-like graphite layer has characteristics similar to the graphene. The graphene-like graphite photodetector demonstrates a narrow photoresponse from 530 to 680 nm, covering visible sources from green to red. The responsivity and external quantum efficiency of the device under the illumination of red laser 660 nm is found to be around 148 mA W−1 and 27.8% respectively and is faster than that reported for similar systems using graphene and reduced graphene oxide previously. A fast response time of 83.7 and 28 µs at a modulation frequency of 1.0 and 10000 Hz respectively from the graphene-like thinly layered graphite photodetector shows potential application for the development of low-cost carbon-based photodetectors in near future.

A significant enhancement in visible-light photodetection properties of chemical spray pyrolysis fabricated CdS thin films by novel Eu doping concentrations

This work deals with the preparation of undoped and rare earth europium-doped cadmium sulfide (Eu:CdS) films by spray pyrolysis procedure. X-ray diffraction (XRD), field emission-scanning electron microscopy, energy dispersive X-ray spectroscopy, ultraviolet-visible spectroscopy and photo-electrical studies were used to analyze the properties of the films. XRD analysis of CdS films showed polycrystalline nature with most preferred orientation along (002) and (101) for Eu:CdS. Eu doping improved the optical properties, the optical band gap of Eu:CdS films is noted to be larger compare to pure CdS thin films. Optical studies reveal the direct band gap variation from 2.43 eV to 2.48 eV with rising Eu doping from 1 wt.% to 5 wt.%. The increase in the nonlinear optical parameter was observed on Eu doping in CdS. The photo-electrical studies done for Eu:CdS films indicates that Eu doping has noticeable influence on electrical properties. The responsitivity for pure was ∼0.0629 AW−1 which increased to 0.614 AW−1, when 5 wt.% Eu was doped. The external quantum efficiency was enriched ∼ 8 times for 5 wt.% Eu:CdS compared to pure. The rise and decay time of Eu:CdS photodetector and its ON/OFF ratio was studied which is noticed ∼10 times larger compared to pure CdS. The observed enhancement in photodetector properties, morphology, crystalline quality and optical properties are caused by Eu doping concentration and hence will be more applicable in optoelectronics.

Highly Transparent and Surface-Plasmon-Enhanced Visible-Photodetector Based on Zinc Oxide Thin-Film Transistors with Heterojunction Structure.

Highly transparent zinc oxide (ZnO)-based thin-film transistors (TFTs) with gold nanoparticles (AuNPs) capable of detecting visible light were fabricated through spray pyrolysis on a fluorine-doped tin oxide substrate. The spray-deposited channel layer of ZnO had a thickness of approximately 15 nm, and the thickness exhibited a linear increase with an increasing number of sprays. Furthermore, the ZnO thin-film exhibited a markedly smoother channel layer with a significantly lower surface roughness of 1.84 nm when the substrate was 20 cm from the spray nozzle compared with when it was 10 cm away. Finally, a ZnO and Au-NP heterojunction nanohybrid structure using plasmonic energy detection as an electrical signal, constitutes an ideal combination for a visible-light photodetector. The ZnO-based TFTs convert localized surface plasmon energy into an electrical signal, thereby extending the wide band-gap of materials used for photodetectors to achieve visible-light wavelength detection. The photo-transistors demonstrate an elevated on-current with an increase of the AuNP density in the concentration of 1.26, 12.6, and 126 pM and reach values of 3.75, 5.18, and 9.79 × 10−7 A with applied gate and drain voltages. Moreover, the threshold voltage (Vth) also drifts to negative values as the AuNP density increases.

Local stress enhanced photocurrent of visible light photo-detection in Cs2AgBiBr6 single crystal

Halide perovskites have attracted worldwide attention in recent years due to their fundamental scientific importance and practical applications in energy harvesting fields. However, its toxicity and instability are hindering the commercial applications of halide perovskite-based devices. Recently discovered inorganic lead-free Cs2AgBiBr6 double perovskite exhibits excellent optical and electronic properties and can be considered as a candidate for photovoltaic devices. In this work, we report the photoelectric performance of the Cs2AgBiBr6 single crystal as a photodetector both macro- and microscopically. By utilizing a nanoscale probe as a top electrode, it is discovered that the photogenerated carriers are effectively collected, as in the case of macroscopic electrodes. More importantly, the photocurrent is enhanced as the applied nanoscale mechanical load increases, which indicates that the local bandgap of the Cs2AgBiBr6 single crystal decreases with the increased nanoscale stress, showing similar tendency with the first-principles calculation result. The reported finding establishes a strategy to enhance the performance of photodetectors, which will be a great help in improving the performance of future optoelectronic devices and energy harvesting devices at the nanoscale.