Understanding Trap States in InP and GaP Quantum Dots through Density Functional Theory.

The widespread application of III-V colloidal quantum dots (QDs) as nontoxic, highly tunable emitters is stymied by their high density of trap states. Here, we utilize density functional theory (DFT) to investigate trap state formation in a diverse set of realistically passivated core-only InP and GaP QDs. Through orbital localization techniques, we deconvolute the dense manifold of trap states to allow for detailed assignment of surface defects. We find that the three-coordinate species dominate trapping in III-V QDs and identify features in the geometry and charge environment of trap centers capable of deepening, or sometimes passivating, traps. Furthermore, we observe stark differences in surface reconstruction between InP and GaP, where the more labile InP reconstructs to passivate three-coordinate indium at the cost of distortion elsewhere. These results offer explanations for experimentally observed trapping behavior and suggest new avenues for controlling trap states in III-V QDs.

Revealing the Band-Edge Exciton Fine Structure of Single InP Nanocrystals.

We investigate the fundamental optical properties of single zinc-blende InP/ZnSe/ZnS nanocrystals (NCs) using frequency- and time-resolved magneto-photoluminescence spectroscopy. At liquid helium temperature, highly resolved spectral fingerprints are obtained and identified as the recombination lines of the three lowest states of the band-edge exciton fine structure. The evolutions of the photoluminescence spectra and decays under magnetic fields show evidence for a ground dark exciton level 0L with zero angular momentum projection along the NC main elongation axis. It lies 300 to 600 µeV below the ±1L bright exciton doublet, which is finely split by the NC shape anisotropy. These spectroscopic findings are well reproduced with a model of exciton fine structure accounting for shape anisotropy of the InP core. Our spectral fingerprints are extremely sensitive to the NC morphologies and unveil highly uniform shapes with prolate deviations of less than 3% from perfect sphericity.

Frustrated Lewis Pair Adduct of Atomic P(-1) as a Source of Phosphinidenes (PR), Diphosphorus (P2 ), and Indium Phosphide.

We report phosphinidenes (PR) stabilized by an intramolecular frustrated Lewis pair (FLP) chelate. These adducts include the parent phosphinidene, PH, which is accessed via thermolysis of coordinated HPCO. The reported FLP-PH species acts as a springboard to other phosphorus-containing compounds, such as FLP-adducts of diphosphorus (P2 ) and InP3 . Our new adducts participate in thermal- or light-induced phosphinidene elimination (of both PH and PR, R=organic group), transfer P2 units to an organic substrate, and yield the useful semiconductor InP at only 110 °C from solution.

Ligand Effect in 1-Octanethiol Passivation of InP/ZnSe/ZnS Quantum Dots-Evidence of Incomplete Surface Passivation during Synthesis.

The lack of anionic carboxylate ligands on the surface of InP/ZnSe/ZnS quantum dots (QDs), where zinc carboxylate ligands can be converted to carboxylic acid or carboxylate ligands via proton transfer by 1-octanethiol, is demonstrated. The as-synthesized QDs initially have an under-coordinated vacancy surface, which is passivated by solvent ligands such as ethanol and acetone. Upon exposure of 1-octanethiol to the QD surface, 1-octanethiol effectively induces the surface binding of anionic carboxylate ligands (derived from zinc carboxylate ligands) by proton transfer, which consequently exchanges ethanol and acetone ligands that bind on the incomplete QD surface. These systematic chemical analyses, such as thermogravimetric analysis-mass spectrometry and proton nuclear magnetic resonance spectroscopy, directly show the interplay of surface ligands, and it associates with QD light-emitting diodes (QD-LEDs). It is believed that this better understanding can lead to industrially feasible QD-LEDs.

Extending the Near-Infrared Emission Range of Indium Phosphide Quantum Dots for Multiplexed In Vivo Imaging.

This report of the reddest emitting indium phosphide quantum dots (InP QDs) to date demonstrates tunable, near-infrared (NIR) photoluminescence (PL) as well as PL multiplexing in the first optical tissue window while avoiding toxic constituents. This synthesis overcomes the InP "growth bottleneck" and extends the emission peak of InP QDs deeper into the first optical tissue window using an inverted QD heterostructure, specifically ZnSe/InP/ZnS core/shell/shell nanoparticles. The QDs exhibit InP shell thickness-dependent tunable emission with peaks ranging from 515-845 nm. The high absorptivity of InP yields effective photoexcitation of the QDs with UV, visible, and NIR wavelengths. These nanoparticles extend the range of tunable direct-bandgap emission from InP-based nanostructures, effectively overcoming a synthetic barrier that has prevented InP-based QDs from reaching their full potential as NIR imaging agents. Multiplexed lymph node imaging in a mouse model demonstrates the potential of the NIR-emitting InP particles for in vivo imaging.

Luminescent Anisotropic Wurtzite InP Nanocrystals.

Indium phosphide (InP) nanocrystals are emerging as an alternative to heavy metal containing nanocrystals for optoelectronic applications but lag behind in terms of synthetic control. Herein, luminescent wurtzite InP nanocrystals with narrow size distribution were synthesized via a cation exchange reaction from hexagonal Cu3P nanocrystals. A comprehensive surface treatment with NOBF4 was performed, which removes excess copper while generating stoichiometric In/P nanocrystals with fluoride surface passivation. The attained InP nanocrystals manifest a highly resolved absorption spectrum with a narrow emission line of 80 meV, and photoluminescence quantum yield of up to 40%. Optical anisotropy measurements on ensemble and single particle bases show the occurrence of polarized transitions directly mirroring the anisotropic wurtzite lattice, as also manifested from modeling of the quantum confined electronic levels. This shows a green synthesis path for achieving wurtzite InP nanocrystals with desired optoelectronic properties including color purity and light polarization with potential for diverse optoelectronic applications.

Engineering Complex Synaptic Behaviors in a Single Device: Emulating Consolidation of Short-term Memory to Long-term Memory in Artificial Synapses via Dielectric Band Engineering.

As one of the key neuronal activities associated with memory in the human brain, memory consolidation is the process of the transition of short-term memory (STM) to long-term memory (LTM), which transforms an external stimulus to permanently stored information. Here, we report the emulation of this complex synaptic function, consolidation of STM to LTM, in a single-crystal indium phosphide (InP) field effect transistor (FET)-based artificial synapse. This behavior is achieved via the dielectric band and charge trap lifetime engineering in a dielectric gate heterostructure of aluminum oxide and titanium oxide. We analyze the behavior of these complex synaptic functions by engineering a variety of action potential parameters, and the devices exhibit good endurance, long retention time (>105 s), and high uniformity. Uniquely, this approach utilizes growth and device fabrication techniques which are scalable and back-end CMOS compatible, making this InP synaptic device a potential building block for neuromorphic computing.

Exploring the Size Limitations of Wurtzite III-V Film Growth.

Metastable crystal phases of abundant semiconductors such as III-Vs, Si, or Ge comprise enormous potential to address current limitations in green light-emitting electrical diodes (LEDs) and group IV photonics. At the same time, these nonconventional polytypes benefit from the chemical similarity to their stable counterparts, which enables the reuse of established processing technology. One of the main challenges is the very limited availability and the small crystal sizes that have been obtained so far. In this work, we explore the limitations of wurtzite (WZ) film epitaxy on the example of InP. We develop a novel method to switch and maintain a metastable phase during a metal-organic vapor phase epitaxy process based on epitaxial lateral overgrowth and compare it with standard selective area epitaxy techniques. We achieve unprecedented large WZ layer dimensions exceeding 100 µm2 and prove their phase purity both by optical as well as structural characterization. On the basis of our observations, we further develop a nucleation-based model and argue on a fundamental size limitation of WZ film growth. Our findings may pave the way toward crystal phase engineered LEDs for highly efficient lighting and display applications.

A Study on an Organic Semiconductor-Based Indirect X-ray Detector with Cd-Free QDs for Sensitivity Improvement.

In this paper, we studied the optimized conditions for adding inorganic quantum dots (QD) to the P3HT:PC70BM organic active layer to increase the sensitivity of the indirect X-ray detector. Commonly used QDs are composed of hazardous substances with environmental problems, so indium phosphide (InP) QDs were selected as the electron acceptor in this experiment. Among the three different sizes of InP QDs (4, 8, and 12 nm in diameter), the detector with 4 nm InP QDs showed the highest sensitivity, of 2.01 mA/Gy·cm2. To further improve the sensitivity, the QDs were fixed to 4 nm in diameter and then the amount of QDs added to the organic active layer was changed from 0 to 5 mg. The highest sensitivity, of 2.26 mA/Gy·cm2, was obtained from the detector with a P3HT:PC70BM:InP QDs (1 mg) active layer. In addition, the highest mobility, of 1.69 × 10-5 cm2/V·s, was obtained from the same detector. Compared to the detector with the pristine P3HT:PC70BM active layer, the detector with a P3HT:PC70BM:InP QDs (1 mg) active layer had sensitivity that was 61.87% higher. The cut-off frequency of the P3HT:PC70BM detector was 21.54 kHz, and that of the P3HT:PC70BM:InP QDs (1 mg) detector was 26.33 kHz, which was improved by 22.24%.

Biocompatible Quantum Funnels for Neural Photostimulation.

Neural photostimulation has high potential to understand the working principles of complex neural networks and develop novel therapeutic methods for neurological disorders. A key issue in the light-induced cell stimulation is the efficient conversion of light to bioelectrical stimuli. In photosynthetic systems developed in millions of years by nature, the absorbed energy by the photoabsorbers is transported via nonradiative energy transfer to the reaction centers. Inspired by these systems, neural interfaces based on biocompatible quantum funnels are developed that direct the photogenerated charge carriers toward the bionanojunction for effective photostimulation. Funnels are constructed with indium-based rainbow quantum dots that are assembled in a graded energy profile. Implementation of a quantum funnel enhances the generated photoelectrochemical current 215% per unit absorbance in comparison with ungraded energy profile in a wireless and free-standing mode and facilitates optical neuromodulation of a single cell. This study indicates that the control of charge transport at nanoscale can lead to unconventional and effective neural interfaces.

Highly Efficient and Bright Inverted Top-Emitting InP Quantum Dot Light-Emitting Diodes Introducing a Hole-Suppressing Interlayer.

InP quantum dots (QDs) based light-emitting diodes (QLEDs) are considered as one of the most promising candidates as a substitute for the environmentally toxic Cd-based QLEDs for future displays. However, the device architecture of InP QLEDs is almost the same as the Cd-based QLEDs even though the properties of Cd-based and InP-based QDs are quite different in their energy levels and shapes. Thus, it is highly required to develop a proper device structure for InP-based QLEDs to improve the efficiency and stability. In this work, efficient, bright, and stable InP/ZnSeS QLEDs based on an inverted top emission QLED (ITQLED) structure by newly introducing a "hole-suppressing interlayer" are demonstrated. The green-emitting ITQLEDs with the hole-suppressing interlayer exhibit a maximum current efficiency of 15.1-21.6 cd A-1 and the maximum luminance of 17 400-38 800 cd m-2 , which outperform the recently reported InP-based QLEDs. The operational lifetime is also increased when the hole-suppressing interlayer is adopted. These superb QLED performances originate not only from the enhanced light-outcoupling by the top emission structure but also from the improved electron-hole balance by introducing a hole-suppressing interlayer which can control the hole injection into QDs.

Concurrent Zinc-Blende and Wurtzite Film Formation by Selection of Confined Growth Planes.

Recent research on nanowires (NWs) demonstrated the ability of III-V semiconductors to adopt a different crystallographic phase when they are grown as nanostructures, giving rise to a novel class of materials with unique properties. Controlling the crystal structure however remains difficult and the geometrical constraints of NWs cause integration challenges for advanced devices. Here, we report for the first time on the phase-controlled growth of micron-sized planar InP films by selecting confined growth planes during template-assisted selective epitaxy. We demonstrate this by varying the orientation of predefined templates, which results in concurrent formation of zinc-blende (ZB) and wurtzite (WZ) material exhibiting phase purities of 100% and 97%, respectively. Optical characterization revealed a 70 meV higher band gap and a 2.5× lower lifetime for WZ InP in comparison to its natural ZB phase. Further, a model for the transition of the crystal structure is presented based on the observed growth facets and the bonding configuration of InP surfaces.

Chemical Structure, Ensemble and Single-Particle Spectroscopy of Thick-Shell InP-ZnSe Quantum Dots.

Thick-shell (>5 nm) InP-ZnSe colloidal quantum dots (QDs) grown by a continuous-injection shell growth process are reported. The growth of a thick crystalline shell is attributed to the high temperature of the growth process and the relatively low lattice mismatch between the InP core and ZnSe shell. In addition to a narrow ensemble photoluminescence (PL) line-width (∼40 nm), ensemble and single-particle emission dynamics measurements indicate that blinking and Auger recombination are reduced in these heterostructures. More specifically, high single-dot ON-times (>95%) were obtained for the core-shell QDs, and measured ensemble biexciton lifetimes, τ2x ∼ 540 ps, represent a 7-fold increase compared to InP-ZnS QDs. Further, high-resolution energy dispersive X-ray (EDX) chemical maps directly show for the first time significant incorporation of indium into the shell of the InP-ZnSe QDs. Examination of the atomic structure of the thick-shell QDs by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) reveals structural defects in subpopulations of particles that may mitigate PL efficiencies (∼40% in ensemble), providing insight toward further synthetic refinement. These InP-ZnSe heterostructures represent progress toward fully cadmium-free QDs with superior photophysical properties important in biological labeling and other emission-based technologies.

Efficient Green Emission from Wurtzite Al xIn1- xP Nanowires.

Direct band gap III-V semiconductors, emitting efficiently in the amber-green region of the visible spectrum, are still missing, causing loss in efficiency in light emitting diodes operating in this region, a phenomenon known as the "green gap". Novel geometries and crystal symmetries however show strong promise in overcoming this limit. Here we develop a novel material system, consisting of wurtzite Al xIn1- xP nanowires, which is predicted to have a direct band gap in the green region. The nanowires are grown with selective area metalorganic vapor phase epitaxy and show wurtzite crystal purity from transmission electron microscopy. We show strong light emission at room temperature between the near-infrared 875 nm (1.42 eV) and the "pure green" 555 nm (2.23 eV). We investigate the band structure of wurtzite Al xIn1- xP using time-resolved and temperature-dependent photoluminescence measurements and compare the experimental results with density functional theory simulations, obtaining excellent agreement. Our work paves the way for high-efficiency green light emitting diodes based on wurtzite III-phosphide nanowires.

Fluorescently labelled multiplex lateral flow immunoassay based on cadmium-free quantum dots.

A sensitive tool for simultaneous qualitative detection of two mycotoxins based on use of non-cadmium quantum dots (QDs) is presented for the first time. QDs have proven themselves as promising fluorescent labels for biolabeling and chemical analysis. With an increasing global tendency to regulate and limit the use of hazardous elements, indium phosphide (InP) QDs are highlighted as environmentally-friendly alternatives to the highly efficient and well-studied, but potentially toxic Cd- and Pb-based QDs. Here, we developed water-soluble InP QDs-based fluorescent nanostructures. They consisted of core/shell InP/ZnS QDs enrobed in a silica shell that allowed the water solubility (QD@SiO2). Then we applied the QD@SiO2 as novel, silica shell-encapsulated fluorescent labels in immunoassays for rapid multiplexed screening. Two mycotoxins, zearalenone and deoxynivalenol, were simultaneously detected in maize and wheat, since the two QD@SiO2 labelled conjugates emit at two different, individually detectable wavelengths. The cutoff values for the simultaneous determination were 50 and 500µgkg-1 for zearalenone and deoxynivalenol, respectively, in both maize and wheat. Liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) was used to confirm the result.

Feasibility of Telecom-Wavelength Photonic Integrated Circuits for Gas Sensors.

To be of commercial interest, gas sensors must optimise, among others, sensitivity, selectivity, longevity, cost and measurement speed. Using the example of ammonia, we establish that integrated optical sensors provide means to maintain the benefits of optical detection set-ups at, in principle, a lower cost and smaller footprint than currently available commercial products. Photonic integrated circuits (PICs) can be used in environmental and agricultural monitoring. The small footprint and great cost scaling of PICs allow for sensor networks with multiple devices. We show, that Indium Phosphide based commercial foundries reached the technological maturity to enable ammonia detection levels at less than 100 ppb. The current unavailability of portable, low cost ammonia sensors with such detection levels prevents emission monitoring, for example, in pig farms. The feasibility of these sensors is investigated by applying the common noise figures of the multiproject wafer platforms operating around 1550 nm to a model for an absorption measurement. The analysis is extended to other relevant gas species with absorption features near telecom-wavelengths.

InP Nanowire Biosensor with Tailored Biofunctionalization: Ultrasensitive and Highly Selective Disease Biomarker Detection.

Electrically active field-effect transistors (FET) based biosensors are of paramount importance in life science applications, as they offer direct, fast, and highly sensitive label-free detection capabilities of several biomolecules of specific interest. In this work, we report a detailed investigation on surface functionalization and covalent immobilization of biomarkers using biocompatible ethanolamine and poly(ethylene glycol) derivate coatings, as compared to the conventional approaches using silica monoliths, in order to substantially increase both the sensitivity and molecular selectivity of nanowire-based FET biosensor platforms. Quantitative fluorescence, atomic and Kelvin probe force microscopy allowed detailed investigation of the homogeneity and density of immobilized biomarkers on different biofunctionalized surfaces. Significantly enhanced binding specificity, biomarker density, and target biomolecule capture efficiency were thus achieved for DNA as well as for proteins from pathogens. This optimized functionalization methodology was applied to InP nanowires that due to their low surface recombination rates were used as new active transducers for biosensors. The developed devices provide ultrahigh label-free detection sensitivities ∼1 fM for specific DNA sequences, measured via the net change in device electrical resistance. Similar levels of ultrasensitive detection of ∼6 fM were achieved for a Chagas Disease protein marker (IBMP8-1). The developed InP nanowire biosensor provides thus a qualified tool for detection of the chronic infection stage of this disease, leading to improved diagnosis and control of spread. These methodological developments are expected to substantially enhance the chemical robustness, diagnostic reliability, detection sensitivity, and biomarker selectivity for current and future biosensing devices.

Effective Surface Passivation of InP Nanowires by Atomic-Layer-Deposited Al2O3 with POx Interlayer.

III/V semiconductor nanostructures have significant potential in device applications, but effective surface passivation is critical due to their large surface-to-volume ratio. For InP such passivation has proven particularly difficult, with substantial depassivation generally observed following dielectric deposition on InP surfaces. We present a novel approach based on passivation with a phosphorus-rich interfacial oxide deposited using a low-temperature process, which is critical to avoid P-desorption. For this purpose we have chosen a POx layer deposited in a plasma-assisted atomic layer deposition (ALD) system at room temperature. Since POx is known to be hygroscopic and therefore unstable in atmosphere, we encapsulate this layer with a thin ALD Al2O3 capping layer to form a POx/Al2O3 stack. This passivation scheme is capable of improving the photoluminescence (PL) efficiency of our state-of-the-art wurtzite (WZ) InP nanowires by a factor of ∼20 at low excitation. If we apply the rate equation analysis advocated by some authors, we derive a PL internal quantum efficiency (IQE) of 75% for our passivated wires at high excitation. Our results indicate that it is more reliable to calculate the IQE as the ratio of the integrated PL intensity at room temperature to that at 10 K. By this means we derive an IQE of 27% for the passivated wires at high excitation (>10 kW cm-2), which constitutes an unprecedented level of performance for undoped InP nanowires. This conclusion is supported by time-resolved PL decay lifetimes, which are also shown to be significantly higher than previously reported for similar wires. The passivation scheme displays excellent long-term stability (>7 months) and is additionally shown to substantially improve the thermal stability of InP surfaces (>300 °C), significantly expanding the temperature window for device processing. Such effective surface passivation is a key enabling technology for InP nanowire devices such as nanolasers and solar cells.

Carrier Recombination Processes in Gallium Indium Phosphide Nanowires.

Understanding of recombination and photoconductivity dynamics of photogenerated charge carriers in GaxIn1-xP NWs is essential for their optoelectronic applications. In this letter, we have studied a series of GaxIn1-xP NWs with varied Ga composition. Time-resolved photoinduced luminescence, femtosecond transient absorption, and time-resolved THz transmission measurements were performed to assess radiative and nonradiative recombination and photoconductivity dynamics of photogenerated charges in the NWs. We conclude that radiative recombination dynamics is limited by hole trapping, whereas electrons are highly mobile until they recombine nonradiatively. We also resolve gradual decrease of mobility of photogenerated electrons assigned to electron trapping and detrapping in a distribution of trap states. We identify that the nonradiative recombination of charges is much slower than the decay of the photoluminescence signal. Further, we conclude that trapping of both electrons and holes as well as nonradiative recombination become faster with increasing Ga composition in GaxIn1-xP NWs. We have estimated early time electron mobility in GaxIn1-xP NWs and found it to be strongly dependent on Ga composition due to the contribution of electrons in the X-valley.

Templated Growth of InP Nanocrystals with a Polytwistane Structure.

We have synthesized InP nanocrystals of an unprecedented crystal phase at low temperature (35-100 °C) by templated growth of InP magic-sized clusters. With the addition of stoichiometric equivalents of P(SiMe3 )3 to the starting cluster, we demonstrate nanocrystal growth mediated through a partial dissolution and recrystallization pathway. This growth process was monitored using a combination of in situ UV/Vis and 31 P NMR spectroscopy, revealing the intermediacy of smaller cluster species of higher symmetry. The nanocrystals that result from this templated growth exhibit a crystal structure that is neither zincblende nor wurtzite, and instead is derived from the original cluster. This structure is best described as a 3D polytwistane phase as deduced from a combination of X-ray diffraction, Raman, and solid-state NMR spectroscopy methods.