Strain modulation in crumpled Si nanomembranes: Light detection beyond the Si absorption limit.

Although Si is extensively used in micro-nano electronics, its inherent optical absorption cutoff at 1100-nm limits its photonic and optoelectronic applications in visible to partly near infrared (NIR) spectral range. Recently, strain engineering has emerged as a promising approach for extending device functionality via tuning the material properties, including change in optical bandgap. In this study, the reduction in bandgap with applied strain was used for extending the absorption limit of crystalline Si up to 1310 nm beyond its intrinsic bandgap, which was achieved by creating the crumpled structures in Si nanomembranes (NMs). The concept was used to develop a prototype NIR image sensor by organizing metal-semiconductor-metal-configured crumpled Si NM photosensing pixels in 6 × 6 array. The geometry-controlled, self-sustained strain induction in Si NMs provided an exclusive photon management with shortening of optical bandgap and enhanced photoresponse beyond the conventional Si absorption limit.

Epitaxial Growth of Wafer-Scale Molybdenum Disulfide/Graphene Heterostructures by Metal-Organic Vapor-Phase Epitaxy and Their Application in Photodetectors.

Van der Waals heterostructures have attracted increasing interest, owing to the combined benefits of their constituents. These hybrid nanostructures can be realized via epitaxial growth, which offers a promising approach for the controlled synthesis of the desired crystal phase and the interface between van der Waals layers. Here, the epitaxial growth of a continuous molybdenum disulfide (MoS2) film on large-area graphene, which was directly grown on a sapphire substrate, is reported. Interestingly, the grain size of MoS2 grown on graphene increases, whereas that of MoS2 grown on SiO2 decreases with an increasing amount of hydrogen in the chemical vapor deposition reactor. In addition, to achieve the same quality, MoS2 grown on graphene requires a much lower growth temperature (400 °C) than that grown on SiO2 (580 °C). The MoS2/graphene heterostructure that was epitaxially grown on a transparent platform was investigated to explore its photosensing properties and was found to exhibit inverse photoresponse with highly uniform photoresponsivity in the photodetector pixels fabricated across a full wafer. The MoS2/graphene heterostructure exhibited ultrahigh photoresponsivity (4.3 × 104 A W-1) upon exposure to visible light of a wide range of wavelengths, confirming the growth of a high-quality MoS2/graphene heterostructure with a clean interface.

Breaking the absorption limit of Si toward SWIR wavelength range via strain engineering.

Silicon has been widely used in the microelectronics industry. However, its photonic applications are restricted to visible and partial near-infrared spectral range owing to its fundamental optical bandgap (1.12 eV). With recent advances in strain engineering, material properties, including optical bandgap, can be tailored considerably. This paper reports the strain-induced shrinkage in the Si bandgap, providing photosensing well beyond its fundamental absorption limit in Si nanomembrane (NM) photodetectors (PDs). The Si-NM PD pixels were mechanically stretched (biaxially) by a maximum strain of ~3.5% through pneumatic pressure-induced bulging, enhancing photoresponsivity and extending the Si absorption limit up to 1550 nm, which is the essential wavelength range of the lidar sensors for obstacle detection in self-driving vehicles. The development of deformable three-dimensional optoelectronics via gas pressure-induced bulging also facilitated the realization of unique device designs with concave and convex hemispherical architectures, which mimics the electronic prototypes of biological eyes.

Superior optical (λ ∼ 1550 nm) emission and detection characteristics of Ge microdisks grown on virtual Si0.5Ge0.5/Si substrates using molecular beam epitaxy.

We report the optical characteristics of relatively large sized (∼7.0-8.0 µm) but low aspect ratio Ge microdisks grown on a virtual Si0.5Ge0.5 substrate using molecular beam epitaxy following the Stranski-Krastanov growth mechanism. Grown microdisks with very low aspect ratio Ge islands exhibit direct band gap (∼0.8 eV) photoluminescence emission sustainable up to room temperature, enabled by the confinement of carriers into the microdisks. p-i-n diodes with an intrinsic layer containing Ge microdisks have been fabricated to study their emission and photoresponse characteristics at an optical communication wavelength of ∼1550 nm. A strong electroluminescence at 1550 nm has been achieved at low temperatures in the device for a very low threshold current density of 2.56 µA cm-2 due to the strong confinement of injected holes. The emission characteristics of the fabricated device with respect to the injected current density and temperature have been studied. Novel emission and optical modulation characteristics at 1550 nm of the fabricated p-i-n device containing Ge microdisks grown on a virtual SiGe substrate indicate its potential for Si CMOS compatible on-chip optical communications.

Direct Synthesis of a Self-Assembled WSe2 /MoS2 Heterostructure Array and its Optoelectrical Properties.

Functional van der Waals heterojunctions of transition metal dichalcogenides are emerging as a potential candidate for the basis of next-generation logic devices and optoelectronics. However, the complexity of synthesis processes so far has delayed the successful integration of the heterostructure device array within a large scale, which is necessary for practical applications. Here, a direct synthesis method is introduced to fabricate an array of self-assembled WSe2 /MoS2 heterostructures through facile solution-based directional precipitation. By manipulating the internal convection flow (i.e., Marangoni flow) of the solution, the WSe2 wires are selectively stacked over the MoS2 wires at a specific angle, which enables the formation of parallel- and cross-aligned heterostructures. The realized WSe2 /MoS2 -based p-n heterojunction shows not only high rectification (ideality factor: 1.18) but also promising optoelectrical properties with a high responsivity of 5.39 A W-1 and response speed of 16 µs. As a feasible application, a WSe2 /MoS2 -based photodiode array (10 × 10) is demonstrated, which proves that the photosensing system can detect the position and intensity of an external light source. The solution-based growth of hierarchical structures with various alignments could offer a method for the further development of large-area electronic and optoelectronic applications.

Ultrasoft silicon nanomembranes: thickness-dependent effective elastic modulus.

For decades, silicon (Si) has been widely used for the mass production of microelectronic circuits. Recently, as the thickness has been reduced to the nanometer scale, its application has expanded to various fields, including flexible and transparent 2D semiconductors. For the reliable and reproducible operation of such large flexible and transparent devices, obtaining precise information about the mechanical properties of low dimensional Si is crucial. Here, we demonstrate that a 2 nm-thick Si nanomembrane (NM) exhibits an extremely low Young's modulus of 3.25 GPa, a two-order smaller value than that of the bulk counterpart. Our systematic measurement of thickness-controlled Si NMs reveals the existence of significant size effect: The effective modulus rapidly changes from 180 GPa to 3.25 GPa under 25 nm to 2 nm thickness reduction. Our theoretical modeling successfully provides physical insight into the unique stiff-to-soft transition and extremely low modulus. We further demonstrate that the modulus of Si NMs can be tailored precisely via the control of surface morphology of membrane. This work therefore provides a comprehensive picture of how and why originally hard & stiff Si deforms so softly in the ultrathin 2D geometry, and proposes a new strategy to design the mechanical properties at nanoscale dimensions.

Controllable P- and N-Type Conversion of MoTe2 via Oxide Interfacial Layer for Logic Circuits.

To realize basic electronic units such as complementary metal-oxide-semiconductor (CMOS) inverters and other logic circuits, the selective and controllable fabrication of p- and n-type transistors with a low Schottky barrier height is highly desirable. Herein, an efficient and nondestructive technique of electron-charge transfer doping by depositing a thin Al2 O3 layer on chemical vapor deposition (CVD)-grown 2H-MoTe2 is utilized to tune the doping from p- to n-type. Moreover, a type-controllable MoTe2 transistor with a low Schottky barrier height is prepared. The selectively converted n-type MoTe2 transistor from the p-channel exhibits a maximum on-state current of 10 µA, with a higher electron mobility of 8.9 cm2 V-1 s-1 at a drain voltage (Vds ) of 1 V with a low Schottky barrier height of 28.4 meV. To validate the aforementioned approach, a prototype homogeneous CMOS inverter is fabricated on a CVD-grown 2H-MoTe2 single crystal. The proposed inverter exhibits a high DC voltage gain of 9.2 with good dynamic behavior up to a modulation frequency of 1 kHz. The proposed approach may have potential for realizing future 2D transition metal dichalcogenide-based efficient and ultrafast electronic units with high-density circuit components under a low-dimensional regime.

Existence of Carbon Nanodots in Human Blood.

Presence of carbon nanostructures (dots of 2-3 nm of diameter) in human blood plasma have been identified for the first time. The observed particles are N-doped carbon dots having surface active oxygen functional groups. This functionalized carbonaceous nanostructure may have been originated through catabolic processes of consumed foods and beverages. It may take part in different catalytic activities of biomolecules in cellular system necessary for normal physiological function which is unexplored yet.

Orientation-dependent optical characterization of atomically thin transition metal ditellurides.

Molybdenum ditellurides (MoTe2) have recently attracted attention owing to their excellent structurally tunable nature between 1T'(metallic)- and 2H(semiconducting)-phases; thus, the controllable fabrication and critical identification of MoTe2 are highly desired. Here, we semi-controllably synthesized 1T'- and 2H-MoTe2 crystals using the atmospheric pressure chemical vapor deposition (APCVD) technique and studied their grain-orientation dependency using polarization-sensitive optical microscopy, Raman scattering, and second-harmonic generation (SHG) microspectroscopy. The polycrystalline 1T'-MoTe2 phase with quasi-1D "Mo-Mo" zigzag chains showed anisotropic optical absorption, leading to a clear visualization of the lattice domains. On the other hand, 2H-MoTe2 lattice grains did not exhibit any discernible difference under polarized light illumination. The combined aforementioned microscopy techniques could be used as an easy-to-access and non-destructive tool for a quick and solid identification of intended lattice orientation development in industry-scale MoTe2 crystal manufacturing.

Superior heterojunction properties of solution processed copper-zinc-tin-sulphide quantum dots on Si.

CZTS nanocrystals have been synthesized via a new facile and environmentally friendly route using olive oil at a relatively low temperature. Nanocrystals synthesized using olive oil have a smaller average size in comparison to those synthesized with a conventional solvent-like ethylenediamine. Nanocrystals with an average diameter of 40, 20 and 6 nm have been extracted from the olive oil at different centrifugation speeds of 500, 1000 and 2000 rpm, respectively. The photovoltaic characteristics of p-CZTS/n-Si heterojunctions fabricated using the synthesized colloidal quaternary nanocrystals are demonstrated. The device fabricated with smallest sized CZTS nanocrystals, having an average diameter of ∼6 nm, exhibits an enhancement in power conversion efficiency of 61% in comparison to that of the device fabricated with the nanocrystals of 40 nm in diameter. A lower reflectance and higher minority carrier life time along with a larger surface-to-volume ratio resulted in an enhanced power conversion efficiency for smaller sized CZTS nanocrystals.

Emission characteristics of self-assembled strained Ge1-xSnx islands for sources in the optical communication region.

Self-assembled strained Ge1-x Sn x islands on Si (100) have been grown at a low temperature using molecular beam epitaxy. The in-built strain and fraction of Sn in the islands have been estimated using x-ray photoelectron spectroscopy and high resolution x-ray diffraction study of grown samples. No-phonon assisted transition in the optical communication wavelength range of 1.4-1.8 µm has been observed in the Ge1-x Sn x island samples. The direct band gap transition intensity is found to increase with a growth in Sn concentration, with this increase in intensity sustained up to a temperature of 130 K in Ge1-x Sn x islands. The observed electroluminescence in p-i-n devices fabricated on Ge1-x Sn x island samples above a threshold bias of 4 V makes them attractive for future Si based optical devices.

One-dimensional Si/Ge nanowires and their heterostructures for multifunctional applications-a review.

Remarkable progress has been made in the field of one-dimensional semiconductor nanostructures for electronic and photonic devices. Group-IV semiconductors and their heterostructures have dominated the years of success in microelectronic industry. However their use in photonic devices is limited since they exhibit poor optical activity due to indirect band gap nature of Si and Ge. Reducing their dimensions below a characteristic length scale of various fundamental parameters like exciton Bohr radius, phonon mean free path, critical size of magnetic domains, exciton diffusion length etc result in the significant modification of bulk properties. In particular, light emission from Si/Ge nanowires due to quantum confinement, strain induced band structure modification and impurity doping may lead to the integration of photonic components with mature silicon CMOS technology in near future. Several promising applications based on Si and Ge nanowires have already been well established and studied, while others are now at the early demonstration stage. The control over various forms of energy and carrier transport through the unconstrained dimension makes Si and Ge nanowires a promising platform to manufacture advanced solid-state devices. This review presents the progress of the research with emphasis on their potential application of Si/Ge nanowires and their heterostructures for electronic, photonic, sensing and energy devices.

Room temperature direct band gap emission characteristics of surfactant mediated grown compressively strained Ge films.

Compressively strained Ge films have been grown on relaxed Si0.45Ge0.55 virtual substrates using molecular beam epitaxy in the presence of Sb as a surfactant. Structural characterization has shown that films grown in the presence of surfactant exhibit very smooth surfaces with a relatively higher strain value in comparison to those grown without any surfactant. The variation of strain with increasing Ge layer thickness was analyzed using Raman spectroscopy. The strain is found to be reduced with increasing film thickness due to the onset of island nucleation following Stranski-Krastanov growth mechanism. No phonon assisted direct band gap photoluminescence from compressively strained Ge films grown on relaxed Si0.45Ge0.55 has been achieved up to room temperature. Excitation power and temperature dependent photoluminescence have been studied in details to investigate the origin of different emission sub-bands.

Novel Colloidal MoS2 Quantum Dot Heterojunctions on Silicon Platforms for Multifunctional Optoelectronic Devices.

Silicon compatible wafer scale MoS2 heterojunctions are reported for the first time using colloidal quantum dots. Size dependent direct band gap emission of MoS2 dots are presented at room temperature. The temporal stability and decay dynamics of excited charge carriers in MoS2 quantum dots have been studied using time correlated single photon counting spectroscopy technique. Fabricated n-MoS2/p-Si 0D/3D heterojunctions exhibiting excellent rectification behavior have been studied for light emission in the forward bias and photodetection in the reverse bias. The electroluminescences with white light emission spectra in the range of 450-800 nm are found to be stable in the temperature range of 10-350 K. Size dependent spectral responsivity and detectivity of the heterojunction devices have been studied. The peak responsivity and detectivity of the fabricated heterojunction detector are estimated to be ~0.85 A/W and ~8 × 10(11) Jones, respectively at an applied bias of -2 V for MoS2 QDs of 2 nm mean diameter. The above values are found to be superior to the reported results on large area photodetector devices fabricated using two dimensional materials.

Enhancement of Efficiency of a Solar Cell Fabricated on Black Si Made by Inductively Coupled Plasma-Reactive Ion Etching Process: A Case Study of a n-CdS/p-Si Heterojunction Cell.

We show that a significant enhancement of solar cell efficiency can be achieved in cells fabricated on black Si made using inductively coupled plasma-reactive ion etching (ICP-RIE). The ICP-RIE-fabricated black Si results in an array of vertically oriented defect-free Si nanocones (average height ∼150 nm; apex diameter ∼25 nm) exhibiting an average reflectance ≤2% over most of the relevant solar spectral range. The enabling role of the ultralow reflectance of the nanostructured black Si has been demonstrated using a heterojunction solar cell fabricated by depositing a n-type CdS film on p-Si nanocones followed by a transparent conducting coating of Al-doped ZnO (AZO). The fabricated n-CdS/p-Si heterojunction exhibits promising power conversion efficiency close to 3%, up from a mere efficient 0.15% for a similar cell fabricated on a planar Si. The effect of the fabrication process for the black Si on solar cell performance has been investigated through the measurements of carrier lifetime and surface recombination velocity. The accompanying model and simulation analysis shows that the conical structure leads to the effective dielectric constant varying smoothly from the value of the air at the top to the value of Si at the base over the length of the nanocone, leading to a substantial reduction of its reflectance.

Fabrication of Si/ZnS radial nanowire heterojunction arrays for white light emitting devices on Si substrates.

Well-separated Si/ZnS radial nanowire heterojunction-based light-emitting devices have been fabricated on large-area substrates by depositing n-ZnS film on p-type nanoporous Si nanowire templates. Vertically oriented porous Si nanowires on p-Si substrates have been grown by metal-assisted chemical etching catalyzed using Au nanoparticles. Isolated Si nanowires with needle-shaped arrays have been made by KOH treatment before ZnS deposition. Electrically driven efficient white light emission from radial heterojunction arrays has been achieved under a low forward bias condition. The observed white light emission is attributed to blue and green emission from the defect-related radiative transition of ZnS and Si/ZnS interface, respectively, while the red arises from the porous surface of the Si nanowire core. The observed white light emission from the Si/ZnS nanowire heterojunction could open up the new possibility to integrate Si-based optical sources on a large scale.

Multifunctional white-light-emitting metal-organic gels with a sensing ability of nitrobenzene.

In this study, three novel luminescent nanofibrous metal-organic gels (MOGs) have been synthesized by the reaction of 1,3,5-tris(3-pyridylmethoxyl)benzene (L) with chloride salts of Cd(II), Hg(II), and Cu(II). The metal-ligand coordination, intermolecular π-π stacking and several other weak interactions found to play an important role in the formation of nanofibrous materials. The gel materials are characterized by rheology, diffuse reflectance spectra and various microscopic techniques such as TEM, FESEM, and AFM. The gels MOG-1 and MOG-2 were found to exhibit significant white photoluminescence, whereas the MOG-3 exhbits green emission upon excitation at 325 nm. Furthermore, the MOG-1 has shown its application as a chemosensor for the remarkable detection of nitroaromatics such as nitrobenzene (NB), 2,4-dinitrophenol (DNP). The significant quenching response for NB and DNP is attributed to the strong charge-transfer interactions between the electron-deficient aromatic ring of NB and the electron rich aromatic group of L in MOG-1. The crystal structure of Cd(II) complex of L reveals the formation one-dimensional network which contains strong π-π interactions within and between the networks and these strong π-π interactions generate the free charge carrier in all these nanofibrous gels.

Optical photoresponse of CuS-n-Si radial heterojunction with Si nanocone arrays fabricated by chemical etching.

The paper deals with the fabrication of a p-CuS-n-Si nanocone heterojunction based highly sensitive broad band photodetector. Cone-like one dimensional Si nanostructures formed by metal assisted chemical etching, with superior antireflection characteristics have been used as templates for fabrication of the heterojunction. Covellite CuS material was synthesized by a simple chemical reaction for used as target material for the fabrication of p-CuS-n-Si nanocone heterojunctions via pulsed laser ablation. The effect of surface texturing of Si (cone like nanostructure vs. planar) on spectral photoresponse and detection is reported.

Transparent and flexible resistive switching memory devices with a very high ON/OFF ratio using gold nanoparticles embedded in a silk protein matrix.

The growing demand for biomaterials for electrical and optical devices is motivated by the need to make building blocks for the next generation of printable bio-electronic devices. In this study, transparent and flexible resistive memory devices with a very high ON/OFF ratio incorporating gold nanoparticles into the Bombyx mori silk protein fibroin biopolymer are demonstrated. The novel electronic memory effect is based on filamentary switching, which leads to the occurrence of bistable states with an ON=OFF ratio larger than six orders of magnitude. The mechanism of this process is attributed to the formation of conductive filaments through silk fibroin and gold nanoparticles in the nanocomposite. The proposed hybrid bio-inorganic devices show promise for use in future flexible and transparent nanoelectronic systems.