16 research outputs found
Optical, electrical, and solar energy-conversion properties of gallium arsenide nanowire-array photoanodes
Periodic arrays of n-GaAs nanowires have been grown by selective-area metalāorganic chemical-vapor deposition on Si and GaAs substrates. The optical absorption characteristics of the nanowire-arrays were investigated experimentally and theoretically, and the photoelectrochemical energy-conversion properties of GaAs nanowire arrays were evaluated in contact with one-electron, reversible, redox species in non-aqueous solvents. The radial semiconductor/liquid junction in the nanowires produced near-unity external carrier-collection efficiencies for nanowire-array photoanodes in contact with non-aqueous electrolytes. These anodes exhibited overall inherent photoelectrode energy-conversion efficiencies of [similar]8.1% under 100 mW cm^ā2 simulated Air Mass 1.5 illumination, with open-circuit photovoltages of 590 Ā± 15 mV and short-circuit current densities of 24.6 Ā± 2.0 mA cm^ā2. The high optical absorption, and minimal reflection, at both normal and off-normal incidence of the GaAs nanowire arrays that occupy <5% of the fractional area of the electrode can be attributed to efficient incoupling into radial nanowire guided and leaky waveguide modes
Toward Optimized Light Utilization in Nanowire Arrays Using Scalable Nanosphere Lithography and Selected Area Growth
Vertically aligned, catalyst-free semiconducting nanowires
hold
great potential for photovoltaic applications, in which achieving
scalable synthesis and optimized optical absorption simultaneously
is critical. Here, we report combining nanosphere lithography (NSL)
and selected area metalāorganic chemical vapor deposition (SA-MOCVD)
for the first time for scalable synthesis of vertically aligned gallium
arsenide nanowire arrays, and surprisingly, we show that such nanowire
arrays with patterning defects due to NSL can be as good as highly
ordered nanowire arrays in terms of optical absorption and reflection.
Wafer-scale patterning for nanowire synthesis was done using a polystyrene
nanosphere template as a mask. Nanowires grown from substrates patterned
by NSL show similar structural features to those patterned using electron
beam lithography (EBL). Reflection of photons from the NSL-patterned
nanowire array was used as a measure of the effect of defects present
in the structure. Experimentally, we show that GaAs nanowires as short
as 130 nm show reflection of <10% over the visible range of the
solar spectrum. Our results indicate that a highly ordered nanowire
structure is not necessary: despite the ādefectsā present
in NSL-patterned nanowire arrays, their optical performance is similar
to ādefect-freeā structures patterned by more costly,
time-consuming EBL methods. Our scalable approach for synthesis of
vertical semiconducting nanowires can have application in high-throughput
and low-cost optoelectronic devices, including solar cells
Red Phosphorus Nanodots on Reduced Graphene Oxide as a Flexible and Ultra-Fast Anode for Sodium-Ion Batteries
Sodium-ion batteries
offer an attractive option for potential low
cost and large scale energy storage due to the earth abundance of
sodium. Red phosphorus is considered as a high capacity anode for
sodium-ion batteries with a theoretical capacity of 2596 mAh/g. However,
similar to silicon in lithium-ion batteries, several limitations,
such as large volume expansion upon sodiation/desodiation and low
electronic conductance, have severely limited the performance of red
phosphorus anodes. In order to address the above challenges, we have
developed a method to deposit red phosphorus nanodots densely and
uniformly onto reduced graphene oxide sheets (P@RGO) to minimize the
sodium ion diffusion length and the sodiation/desodiation stresses,
and the RGO network also serves as electron pathway and creates free
space to accommodate the volume variation of phosphorus particles.
The resulted P@RGO flexible anode achieved 1165.4, 510.6, and 135.3
mAh/g specific charge capacity at 159.4, 31878.9, and 47818.3 mA/g
charge/discharge current density in rate capability test, and a 914
mAh/g capacity after 300 deep cycles in cycling stability test at
1593.9 mA/g current density, which marks a significant performance
improvement for red phosphorus anodes for sodium-ion chemistry and
flexible power sources for wearable electronics
Electrical and Optical Characterization of Surface Passivation in GaAs Nanowires
We report a systematic study of carrier dynamics in Al<sub><i>x</i></sub>Ga<sub>1ā<i>x</i></sub>As-passivated
GaAs nanowires. With passivation, the minority carrier diffusion length
(<i>L</i><sub>diff</sub>) increases from 30 to 180 nm, as
measured by electron beam induced current (EBIC) mapping, and the
photoluminescence (PL) lifetime increases from sub-60 ps to 1.3 ns.
A 48-fold enhancement in the continuous-wave PL intensity is observed
on the same individual nanowire with and without the Al<sub><i>x</i></sub>Ga<sub>1ā<i>x</i></sub>As passivation
layer, indicating a significant reduction in surface recombination.
These results indicate that, in passivated nanowires, the minority
carrier lifetime is not limited by twin stacking faults. From the
PL lifetime and minority carrier diffusion length, we estimate the
surface recombination velocity (SRV) to range from 1.7 Ć 10<sup>3</sup> to 1.1 Ć 10<sup>4</sup> cmĀ·s<sup>ā1</sup>, and the minority carrier mobility Ī¼ is estimated to lie in
the range from 10.3 to 67.5 cm<sup>2</sup> V<sup>ā1</sup> s<sup>ā1</sup> for the passivated nanowires
GaAs Nanowire Array Solar Cells with Axial pāiān Junctions
Because of unique structural, optical,
and electrical properties,
solar cells based on semiconductor nanowires are a rapidly evolving
scientific enterprise. Various approaches employing IIIāV nanowires
have emerged, among which GaAs, especially, is under intense research
and development. Most reported GaAs nanowire solar cells form pān
junctions in the radial direction; however, nanowires using axial
junction may enable the attainment of high open circuit voltage (<i>V</i><sub>oc</sub>) and integration into multijunction solar
cells. Here, we report GaAs nanowire solar cells with axial pāiān
junctions that achieve 7.58% efficiency. Simulations show that axial
junctions are more tolerant to doping variation than radial junctions
and lead to higher <i>V</i><sub>oc</sub> under certain conditions.
We further study the effect of wire diameter and junction depth using
electrical characterization and cathodoluminescence. The results show
that large diameter and shallow junctions are essential for a high
extraction efficiency. Our approach opens up great opportunity for
future low-cost, high-efficiency photovoltaics
A two-dimensional mid-infrared optoelectronic retina enabling simultaneous perception and encoding
Designing an infrared machine vision system that can efficiently perceive, convert, and process a massive amount of data remains a challenge. Here, the authors present a retina-inspired 2D optoelectronic device based on van der Waals heterostructure that can perform the data perception and spike-encoding simultaneously for night vision, sensing, spectroscopy, and free-space communications
A two-dimensional mid-infrared optoelectronic retina enabling simultaneous perception and encoding
Infrared machine vision system for object perception and recognition is becoming increasingly important in the Internet of Things era. However, the current system suffers from bulkiness and inefficiency as compared to the human retina with the intelligent and compact neural architecture. Here, we present a retina-inspired mid-infrared (MIR) optoelectronic device based on a two-dimensional (2D) heterostructure for simultaneous data perception and encoding. A single device can perceive the illumination intensity of a MIR stimulus signal, while encoding the intensity into a spike train based on a rate encoding algorithm for subsequent neuromorphic computing with the assistanceofanall-opticalexcitationmechanism, a stochastic near-infrared (NIR) sampling terminal. The device features wide dynamic working range, high encoding precision, and flexible adaption ability to the MIR intensity. Moreover, an inference accuracy more than 96% toMIR MNIST data set encoded by the device is achieved using a trained spiking neural network (SNN).Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)National Medical Research Council (NMRC)National Research Foundation (NRF)Published versionThis work was supported by the Singapore Ministry of Education (MOE-T2EP50120-0009 (Q.J.W.)), Agency for Science, Technology and Research (A*STAR) (A18A7b0058 (Q.J.W.) and A2090b0144 (Q.J.W.)), National Medical Research Council (NMRC) (021528- 00001 (Q.J.W.)), and National Research Foundation Singapore (NRF-CRP22-2019-0007 (Q.J.W.)), National Key Research and Development Program of China (2022YFB2802803 (N.C.)), the Natural Science Foundation of China Project (61925104 (N.C.), 62031011 (N.C.)) and Major Key Project of PCL (N.C.), and F.H. acknowledges the support from the China Scholarship Council