Jinyou Xu

Growth of Alloy MoS2xSe2(1–x) Nanosheets with Fully Tunable Chemical Compositions and Optical Properties

Band gap engineering of atomically thin two-dimensional layered materials is critical for their applications in nanoelectronics, optoelectronics, and photonics. Here we report, for the first time, a simple one-step chemical vapor deposition approach for the simultaneous growth of alloy MoS2xSe2(1-x) triangular nanosheets with complete composition tunability. Both the Raman and the photoluminescence studies show tunable optical properties consistent with composition of the alloy nanosheets. Importantly, all samples show a single bandedge emission peak, with the spectral peak position shifting from 668 nm (for pure MoS2) to 795 nm (for pure MoSe2), indicating the high quality for these complete composition alloy nanosheets. These band gap engineered 2D structures could open up an exciting opportunity for probing their fundamental physical properties in 2D and may find diverse applications in functional electronic/optoelectronic devices.

Spatial bandgap engineering along single alloy nanowires.

Bandgap engineering of semiconductor nanowires is important in designing nanoscale multifunctional optoelectronic devices. Here, we report a facile thermal evaporation method, and realize the spatial bandgap engineering in single CdS(1-x)Se(x) alloy nanowires. Along the length of these achieved nanowires, the composition can be continuously tuned from x = 0 (CdS) at one end to x = 1 (CdSe) at the other end, resulting in the corresponding bandgap (light emission wavelength) being modulated gradually from 2.44 eV (507 nm, green light) to 1.74 eV (710 nm, red light). In spite of the existing composition (crystal lattice) transition along the length, these multicolor nanowires still possess high-quality crystallization. These bandgap engineered nanowires will have promising applications in such as multicolor display and lighting, high-efficiency solar cells, ultrabroadly spectral detectors, and biotechnology.

Room-Temperature Near-Infrared Photodetectors Based on Single Heterojunction Nanowires

Nanoscale near-infrared photodetectors are attractive for their potential applications in integrated optoelectronic devices. Here we report the synthesis of GaSb/GaInSb p-n heterojunction semiconductor nanowires for the first time through a controllable chemical vapor deposition (CVD) route. Based on these nanowires, room-temperature, high-performance, near-infrared photodetectors were constructed. The fabricated devices show excellent light response in the infrared optical communication region (1.55 μm), with an external quantum efficiency of 10(4), a responsivity of 10(3) A/W, and a short response time of 2 ms, which shows promising potential applications in integrated photonics and optoelectronics devices or systems.

Room-temperature dual-wavelength lasing from single-nanoribbon lateral heterostructures.

Nanoscale dual-wavelength lasers are attractive for their potential applications in highly integrated photonic devices. Here we report the growth of nanoribbon lateral heterostructures made of a CdS(x)Se(1-x) central region with epitaxial CdS lateral sides using a multistep thermal evaporation route with a moving source. Under laser excitation, the emission of these ribbons indicates sandwich-like structures along the width direction, with characteristic red emission in the center and green emission at both edges. More importantly, dual-wavelength lasing with tunable wavelengths is demonstrated at room temperature based on these single-nanoribbon heterostructures for the first time. These achievements represent a significant advance in designing nanoscale dual-wavelength lasers and have the potential to open up new and exciting opportunities for diverse applications in integrated photonics, optoelectronics, and sensing.

On-Nanowire Spatial Band Gap Design for White Light Emission

We demonstrated a substrate-moving vapor-liquid-solid (VLS) route for growing composition gradient ZnCdSSe alloy nanowires. Relying on temperature-selected composition deposition along their lengths, single tricolor ZnCdSSe alloy nanowires with engineerable band gap covering the entire visible range were obtained. The photometric property of these tricolor nanowires, which was determined by blue-, green-, and red-color emission intensities, can be in turn controlled by their corresponding emission lengths. More particularly, under carefully selected growth conditions, on-nanowire white light emission has been achieved. Band-gap-engineered semiconductor alloy nanowires demonstrated here may find applications in broad band light absorption and emission devices.

Wavelength-Converted/Selective Waveguiding Based on Composition-Graded Semiconductor Nanowires

Compact wavelength-sensitive optical components are desirable for optical information processing and communication in photonic integrated system. In this work, optical waveguiding along single composition-graded CdS(x)Se(1-x) nanowires were systematically investigated. Under a focused laser excitation, the excited light can be guided passively along the bandgap-increased direction of the nanowire, keeping the photonic energy of the guided light almost unchanged during the whole propagation. In comparison, the excited light is guided actively through incessantly repeated band-to-band reabsorption and re-emitting processes along the bandgap-decreased direction, resulting in a gradual wavelength conversion during propagation. On the basis of this wavelength-converted waveguiding, a concept of nanoscale wavelength splitter is demonstrated by assembling a graded nanowire with several composition-uniform nanowires into branched nanowire structure. Our study indicates that composition-graded semiconductor nanowires would open new exciting opportunities in developing new wavelength-sensitive optical components for integrated nanophotonic devices.

Low-Threshold Nanowire Laser Based on Composition-Symmetric Semiconductor Nanowires

Low-threshold nanoscale lasers are attractive for their promising applications in highly integrated photonic devices and systems. Here we report the controllable growth of composition-symmetric CdS(x)Se(1-x) nanowires by using a multistep thermal evaporation route with moving sources. Microstructure analyses reveal the obtained wires are high-quality single crystals with the composition gradually changed from the center toward their both ends. Under laser illumination, these wires exhibit symmetrical color distribution along the length direction, with red at the center and green at the both ends. Optically pumped lasing is realized at room temperature using these composition-symmetric nanowires, with the threshold several times lower than that of composition-homogeneous wires. This new nanowire structure will have potential applications as low-threshold nanoscale lasers in integrated nanophotonics.

Asymmetric light propagation in composition-graded semiconductor nanowires

Asymmetric light propagation is crucial to the development of optical-based functional components in nanophotonics. Diverse configurations and structures have been proposed to allow asymmetrical propagation of photonic signal, but on-chip integration is difficult to achieve due to their complex structure and/or relatively large footprint. Here we report the first design and realization of asymmetric light propagation in single semiconductor nanowires with a composition gradient along the length. We show the asymmetric nanowire waveguides can be synthesized using a simple thermal evaporation and vapor transport approach without involving complicated and costly fabrication processes. Our studies demonstrate the asymmetric nanowire waveguides offer some significant advantages over previous designs, including ultra-low operation power, tunable working wavelength and nanoscale footprint, making them attractive building blocks for integrated photonic circuits.

Semiconductor Alloy Nanoribbon Lateral Heterostructures for High‐Performance Photodetectors

High-performance visible-light photodetectors are achieved based on single nanoribbon lateral heterostructures composed of two different semiconductor alloys epitaxially grown in the lateral direction. They reveal superior spectral response range, responsivity, Ion/Ioff ratio, and external quantum efficiency, relative to devices based on single composition structures.

Trap-state whispering-gallery mode lasing from high-quality tin-doped CdS whiskers

High-quality surface tin-doped hexagonal CdS whiskers were synthesized by a well-controlled in situ source exchange chemical vapor deposition route. Under local light excitation, the detected microphotoluminescence from any positions along the length of these whiskers exhibits strong and multi-mode trap-state whispering-gallery (WG) mode emission. With elevating the pumping power, some of these WG modes start lasing at an ultra-low threshold, with lasing wavelength covering a broad range from ∼540 to ∼750 nm. Calculations using a plane-wave model of WG modes show that all these trap-state lasing modes are transverse magnetic polarized, which was well explained by a two-dimensional finite element simulation. The surface doped semiconductor structures have potential applications as low-threshold tunable micro/nanoscale lasers in optical storage, lighting, and optical communications.High-quality surface tin-doped hexagonal CdS whiskers were synthesized by a well-controlled in situ source exchange chemical vapor deposition route. Under local light excitation, the detected microphotoluminescence from any positions along the length of these whiskers exhibits strong and multi-mode trap-state whispering-gallery (WG) mode emission. With elevating the pumping power, some of these WG modes start lasing at an ultra-low threshold, with lasing wavelength covering a broad range from ∼540 to ∼750 nm. Calculations using a plane-wave model of WG modes show that all these trap-state lasing modes are transverse magnetic polarized, which was well explained by a two-dimensional finite element simulation. The surface doped semiconductor structures have potential applications as low-threshold tunable micro/nanoscale lasers in optical storage, lighting, and optical communications.