Semiconductor nanowires represent an important class of nanostructure building block for photovoltaics as well as direct solar-to-fuel application because of their high surface area, tunable bandgap and efficient charge transport and collection. In this talk, I will highlight several recent examples in this lab using semiconductor nanowires and their heterostructures for the purpose of solar energy harvesting. In addition, we have also discovered that the thermoconductivity of the silicon nanowires can be significantly reduced due to phonon scattering, pointing to a very promising approach to design better thermoelectrical materials. It is important to note that the engines that generate most of the world's power typically operate at only 30–40 per cent efficiency, releasing roughly 15 terawatts of heat to the environment. If this “wasted heat” could be recycled, the impact globally would be enormous. Our silicon nanowire thermoelectric technology could have a significant impact in alternative energy generation.

Semiconductor nanowires for energy conversion.

Photovoltaics based on nanowire arrays could reduce cost and materials consumption compared with planar devices but have exhibited low efficiency of light absorption and carrier collection. We fabricated a variety of millimeter-sized arrays of p-type/intrinsic/n-type (p-i-n) doped InP nanowires and found that the nanowire diameter and the length of the top n-segment were critical for cell performance. Efficiencies up to 13.8% (comparable to the record planar InP cell) were achieved by using resonant light trapping in 180-nanometer-diameter nanowires that only covered 12% of the surface. The share of sunlight converted into photocurrent (71%) was six times the limit in a simple ray optics description. Furthermore, the highest open-circuit voltage of 0.906 volt exceeds that of its planar counterpart, despite about 30 times higher surface-to-volume ratio of the nanowire cell.

https://science.sciencemag.org/content/sci/339/6123/1057.full.pdf

InP Nanowire Array Solar Cells Achieving 13.8% Efficiency by Exceeding the Ray Optics Limit

Whereas standard transmission electron microscopy studies are unable to preserve the native state of chemically reactive and beam-sensitive battery materials after operation, such materials remain pristine at cryogenic conditions. It is then possible to atomically resolve individual lithium metal atoms and their interface with the solid electrolyte interphase (SEI). We observe that dendrites in carbonate-based electrolytes grow along the (preferred), , or directions as faceted, single-crystalline nanowires. These growth directions can change at kinks with no observable crystallographic defect. Furthermore, we reveal distinct SEI nanostructures formed in different electrolytes.

Atomic structure of sensitive battery materials and interfaces revealed by cryo–electron microscopy

In this perspective, we take a critical look at the research progress within the nanowire community for the past decade We discuss issues on the discovery of fundamentally new phenomena versus performance benchmarking for many of the nanowire applications We also notice that both the bottom-up and top-down approaches have played important roles in advancing our fundamental understanding of this new class of nanostructures Finally we attempt to look into the future and offer our personal opinions on what the future trends will be in nanowire research

/pdf/semiconductor-nanowire-what-s-next-3b24320mit.pdf

Semiconductor nanowire: what's next?

The manipulation of optical energy in structures smaller than the wavelength of light is key to the development of integrated photonic devices for computing, communications and sensing. Wide band gap semiconductor nanostructures with near-cylindrical geometry and large dielectric constants exhibit two-dimensional ultraviolet and visible photonic confinement (i.e. waveguiding). Combined with optical gain, the waveguiding behavior facilitates highly directional lasing at room temperature in controlled-growth nanowires with favorable resonant feedback. We have further explored the properties and functions of individual ultralong crystalline oxide nanoribbons that act as subwavelength optical waveguides, nonlinear frequency converter and assess their applicability as nanoscale photonic elements and scanning probes. Semiconductor nanowires offer a versatile photonic platform due to the ability to specify material size, shape, and composition. The integration of multiple unique materials with distinct optical properties promises to enable advances for several applications ranging from solid state lighting, biochemical sensing to imaging and spectroscopy.

http://ieeexplore.ieee.org/iel5/4302825/4302826/04302854.pdf

Nanowire Photonics

We present results of ideal epitaxial nucleation and growth of III−V semiconductor nanowires on silicon substrates. This addresses the long-time challenge of integrating high performance III−V semiconductors with mainstream Si technology. Efficient room-temperature generation of light on silicon is demonstrated by the incorporation of double heterostructure segments in such nanowires. We expect that advanced heterostructure devices, such as resonant tunneling diodes, superlattice device structures, and heterostructure photonic devices for on-chip communication, could now become available as complementary device technologies for integration with silicon.

Epitaxial III-V nanowires on silicon

Controlled growth of nanowires is an important, emerging research field with many applications in, for example, electronics, photonics, and life sciences. Nanowires of zinc blende crystal structure, grown in the 〈111〉B direction, which is the favoured direction of growth, usually have a large number of twin-plane defects. Such defects limit the performance of optoelectronic nanowire-based devices. To investigate this defect formation, we examine GaP nanowires grown by metal-organic vapour-phase epitaxy. We show that the nanowire segments between the twin planes are of octahedral shape and are terminated by {111} facets, resulting in a microfaceting of the nanowires. We discuss these findings in a nucleation context, where we present an idea on how the twin planes form. This investigation contributes to the understanding of defect formation in nanowires. One future prospect of such knowledge is to determine strategies on how to control the crystallinity of nanowires.

Structural properties of 〈111〉B -oriented III–V nanowires

Growth of one-dimensional nanostructures in MOVPE

Vertical light emitting diodes (LEDs) based on GaAs/InGaP core/shell nanowires, epitaxially grown on GaP and Si substrates, have been fabricated. The devices can be fabricated over large areas and can be precisely positioned on the substrates, by the use of standard lithography techniques, enabling applications such as on-chip optical communication. LED functionality was established on both kinds of substrate, and the devices were evaluated in terms of temperature-dependent photoluminescence and electroluminescence.

https://iopscience.iop.org/article/10.1088/0957-4484/19/30/305201/pdf

C. Patrik T. Svensson

Papers

Epitaxial III-V nanowires on silicon

Structural properties of 〈111〉B -oriented III–V nanowires

Growth of one-dimensional nanostructures in MOVPE

Monolithic GaAs/InGaP nanowire light emitting diodes on silicon

The structure of B oriented GaP nanowires