InN/InGaN Quantum Dot Abiotic One-Compartment Glucose Photofuel Cell: Power Supply and Sensing

InN/InGaN quantum dots (QDs) are introduced as an efficient photoanode for a novel abiotic one-compartment photofuel cell (PFC) with a Pt cathode and glucose as a biofuel. Due to the high catalytic activity and selectivity of the InN/InGaN QDs toward oxidation reactions, the PFC operates without a membrane under physiologically mild conditions at medium to low glucose concentrations with a noble-metal-free photoanode. A relatively high short-circuit photocurrent density of 0.56 mA/cm² and a peak output power density of 0.22 mW/cm² are achieved under 1 sun illumination for a 0.1 M glucose concentration with optimized InN/InGaN QDs of the right size. The super-linear dependence of the short-circuit photocurrent density and the output power density as a function of the logarithmic glucose concentration makes the PFC well suited for sensing, covering the 4-6 mM range of glucose concentration in blood under normal conditions with good selectivity. No degradation of the PFC operation over time is observed.

Metalorganic chemical vapor deposition of InN quantum dots and nanostructures

Using one material system from the near infrared into the ultraviolet is an attractive goal, and may be achieved with (In,Al,Ga)N. This III-N material system, famous for enabling blue and white solid-state lighting, has been pushing towards longer wavelengths in more recent years. With a bandgap of about 0.7 eV, InN can emit light in the near infrared, potentially overlapping with the part of the electromagnetic spectrum currently dominated by III-As and III-P technology. As has been the case in these other III-V material systems, nanostructures such as quantum dots and quantum dashes provide additional benefits towards optoelectronic devices. In the case of InN, these nanostructures have been in the development stage for some time, with more recent developments allowing for InN quantum dots and dashes to be incorporated into larger device structures. This review will detail the current state of metalorganic chemical vapor deposition of InN nanostructures, focusing on how precursor choices, crystallographic orientation, and other growth parameters affect the deposition. The optical properties of InN nanostructures will also be assessed, with an eye towards the fabrication of optoelectronic devices such as light-emitting diodes, laser diodes, and photodetectors.

Comparison of the Extended Gate Field-Effect Transistor with Direct Potentiometric Sensing for Super-Nernstian InN/InGaN Quantum Dots

We systematically study the sensitivity and noise of an InN/InGaN quantum dot (QD) extended gate field-effect transistor (EGFET) with super-Nernstian sensitivity and directly compare the performance with potentiometric sensing. The QD sensor exhibits a sensitivity of -80 mV/decade with excellent linearity over a wide concentration range, assessed for chloride anion detection in 10^-4 to 0.1 M KCl aqueous solutions. The sensitivity and linearity are reproduced for the EGFET and direct open-circuit potential (OCP) readout. The EGFET noise in the saturated regime is smaller than the OCP noise, while the EGFET noise in the linear regime is largest. This highlights EGFET operation in the saturated regime for most precise measurements and the lowest limit of detection and the lowest limit of quantification, which is attributed to the low-impedance current measurement at a relatively high bias and the large OCP for the InN/InGaN QDs.

Single-nanostructure bandgap engineering enabled by magnetic-pulling thermal evaporation growth

Realizing the substantial potential of bottom-up 1D semiconductor nanostructures in developing functional nanodevices calls for dedicated single-nanostructure bandgap engineering by various growth approaches. Although thermal evaporation has been advised as a facile approach for most semiconductors to form 1D nanostructures from bottom-up, its capability of achieving single-nanostructure bandgap engineering was considered a challenge. In 2011, we succeeded in the direct growth of composition-graded CdS1-x Se x (0 ≤ x ≤ 1) nanowires by upgrading the thermal-evaporation tube furnace with a home-made magnetic-pulling module. This report aims to provide a comprehensive review of the latest advances in the single-nanostructure bandgap engineering enabled by the magnetic-pulling thermal evaporation growth. The report begins with the description of different magnetic-pulling thermal evaporation strategies associated with diverse examples of composition-engineered 1D nanostructures. Following is an elaboration on their optoelectronic applications based on the resulting single-nanostructure bandgap engineering, including monolithic white-light sources, proof-of-concept asymmetric light propagation and wavelength splitters, monolithic multi-color and white-light lasers, broadband-response photodetectors, high-performance transistors, and recently the most exciting single-nanowire spectrometer. In the end, this report concludes with some personal perspectives on the directions toward which future research might be advanced.

Paving the road toward the use of β-Fe2O3 in solar water splitting: Raman identification, phase transformation and strategies for phase stabilization

Although β-Fe2O3 has a high theoretical solar-to-hydrogen efficiency because of its narrow band gap, the study of β-Fe2O3 photoanodes for water splitting is elusive as a result of their metastable nature. Raman identification of β-Fe2O3 is theoretically and experimentally investigated in this study for the first time, thus clarifying the debate about its Raman spectrum in the literature. Phase transformation of β-Fe2O3 to α-Fe2O3 was found to potentially take place under laser and electron irradiation as well as annealing. Herein, phase transformation of β-Fe2O3 to α-Fe2O3 was inhibited by introduction of Zr doping, and β-Fe2O3 was found to withstand a higher annealing temperature without any phase transformation. The solar water splitting photocurrent of the Zr-doped β-Fe2O3 photoanode was increased by 500% compared to that of the pure β-Fe2O3 photoanode. Additionally, Zr-doped β-Fe2O3 exhibited very good stability during the process of solar water splitting. These results indicate that by improving its thermal stability, metastable β-Fe2O3 film is a promising photoanode for solar water splitting.

Electric dipole of InN/InGaN quantum dots and holes and giant surface photovoltage directly measured by Kelvin probe force microscopy

We directly measure the electric dipole of InN quantum dots (QDs) grown on In-rich InGaN layers by Kelvin probe force microscopy. This significantly advances the understanding of the superior catalytic performance of InN/InGaN QDs in ion- and biosensing and in photoelectrochemical hydrogen generation by water splitting and the understanding of the important third-generation InGaN semiconductor surface in general. The positive surface photovoltage (SPV) gives an outward QD dipole with dipole potential of the order of 150 mV, in agreement with previous calculations. After HCl-etching, to complement the determination of the electric dipole, a giant negative SPV of -2.4 V, significantly larger than the InGaN bandgap energy, is discovered. This giant SPV is assigned to a large inward electric dipole, associated with the appearance of holes, matching the original QD lateral size and density. Such surprising result points towards unique photovoltaic effects and photosensitivity.

Spatial Surface Charge Engineering for Electrochemical Electrodes

We introduce a novel concept for the design of functional surfaces of materials: Spatial surface charge engineering. We exploit the concept for an all-solid-state, epitaxial InN/InGaN-on-Si reference electrode to replace the inconvenient liquid-filled reference electrodes, such as Ag/AgCl. Reference electrodes are universal components of electrochemical sensors, ubiquitous in electrochemistry to set a constant potential. For subtle interrelation of structure design, surface morphology and the unique surface charge properties of InGaN, the reference electrode has less than 10 mV/decade sensitivity over a wide concentration range, evaluated for KCl aqueous solutions and less than 2 mV/hour long-time drift over 12 hours. Key is a nanoscale charge balanced surface for the right InGaN composition, InN amount and InGaN surface morphology, depending on growth conditions and layer thickness, which is underpinned by the surface potential measured by Kelvin probe force microscopy. When paired with the InN/InGaN quantum dot sensing electrode with super-Nernstian sensitivity, where only structure design and surface morphology are changed, this completes an all-InGaN-based electrochemical sensor with unprecedented performance.