Rapid progress in the synthesis and processing of materials with structure on nanometer length scales has created a demand for greater scientific understanding of thermal transport in nanoscale devices, individual nanostructures, and nanostructured materials. This review emphasizes developments in experiment, theory, and computation that have occurred in the past ten years and summarizes the present status of the field. Interfaces between materials become increasingly important on small length scales. The thermal conductance of many solid–solid interfaces have been studied experimentally but the range of observed interface properties is much smaller than predicted by simple theory. Classical molecular dynamics simulations are emerging as a powerful tool for calculations of thermal conductance and phonon scattering, and may provide for a lively interplay of experiment and theory in the near term. Fundamental issues remain concerning the correct definitions of temperature in nonequilibrium nanoscale systems. Modern Si microelectronics are now firmly in the nanoscale regime—experiments have demonstrated that the close proximity of interfaces and the extremely small volume of heat dissipation strongly modifies thermal transport, thereby aggravating problems of thermal management. Microelectronic devices are too large to yield to atomic-level simulation in the foreseeable future and, therefore, calculations of thermal transport must rely on solutions of the Boltzmann transport equation; microscopic phonon scattering rates needed for predictive models are, even for Si, poorly known. Low-dimensional nanostructures, such as carbon nanotubes, are predicted to have novel transport properties; the first quantitative experiments of the thermal conductivity of nanotubes have recently been achieved using microfabricated measurement systems. Nanoscale porosity decreases the permittivity of amorphous dielectrics but porosity also strongly decreases the thermal conductivity. The promise of improved thermoelectric materials and problems of thermal management of optoelectronic devices have stimulated extensive studies of semiconductor superlattices; agreement between experiment and theory is generally poor. Advances in measurement methods, e.g., the 3ω method, time-domain thermoreflectance, sources of coherent phonons, microfabricated test structures, and the scanning thermal microscope, are enabling new capabilities for nanoscale thermal metrology.

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Nanoscale thermal transport

Interpretation of europium(III) spectra

Photovoltaic (PV) technologies for solar energy conversion represent promising routes to green and renewable energy generation. Despite relevant PV technologies being available for more than half a century, the production of solar energy remains costly, largely owing to low power conversion efficiencies of solar cells. The main difficulty in improving the efficiency of PV energy conversion lies in the spectral mismatch between the energy distribution of photons in the incident solar spectrum and the bandgap of a semiconductor material. In recent years, luminescent materials, which are capable of converting a broad spectrum of light into photons of a particular wavelength, have been synthesized and used to minimize the losses in the solar-cell-based energy conversion process. In this review, we will survey recent progress in the development of spectral converters, with a particular emphasis on lanthanide-based upconversion, quantum-cutting and down-shifting materials, for PV applications. In addition, we will also present technical challenges that arise in developing cost-effective high-performance solar cells based on these luminescent materials.

Enhancing solar cell efficiency: the search for luminescent materials as spectral converters

A diverse spectrum of technology drivers such as improved thermal barriers, higher efficiency thermoelectric energy conversion, phase-change memory, heat-assisted magnetic recording, thermal management of nanoscale electronics, and nanoparticles for thermal medical therapies are motivating studies of the applied physics of thermal transport at the nanoscale. This review emphasizes developments in experiment, theory, and computation in the past ten years and summarizes the present status of the field. Interfaces become increasingly important on small length scales. Research during the past decade has extended studies of interfaces between simple metals and inorganic crystals to interfaces with molecular materials and liquids with systematic control of interface chemistry and physics. At separations on the order of ∼1 nm, the science of radiative transport through nanoscale gaps overlaps with thermal conduction by the coupling of electronic and vibrational excitations across weakly bonded or rough interface...

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Nanoscale thermal transport. II. 2003–2012

Understanding energy dissipation and transport in nanoscale structures is of great importance for the design of energy-efficient circuits and energy-conversion systems. This is also a rich domain for fundamental discoveries at the intersection of electron, lattice (phonon), and optical (photon) interactions. This review presents recent progress in understanding and manipulation of energy dissipation and transport in nanoscale solid-state structures. First, the landscape of power usage from nanoscale transistors (∼10−8 W) to massive data centers (∼109 W) is surveyed. Then, focus is given to energy dissipation in nanoscale circuits, silicon transistors, carbon nanostructures, and semiconductor nanowires. Concepts of steady-state and transient thermal transport are also reviewed in the context of nanoscale devices with sub-nanosecond switching times. Finally, recent directions regarding energy transport are reviewed, including electrical and thermal conductivity of nanostructures, thermal rectification, and the role of ubiquitous material interfaces. 
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Energy dissipation and transport in nanoscale devices

Concentration dependence of Mn(2+) luminescence in NaCaPO(4)/Mn(2+) is investigated by structural analyses and optical and laser excitation spectroscopies in the temperature range 19-300 K. NaCaPO(4)/Mn(2+) forms solid solution over the Mn(2+) concentration range 1.0-22 mol %. We observe the red shift and unusual band narrowing of Mn(2+) emission by increasing Mn(2+) concentration in NaCaPO(4). The lifetime of Mn(2+) emission lengthens unexpectedly for higher Mn(2+) concentration. The results are discussed in relation with crystal structure, photon reabsorption, exchange interaction, and energy transfer and energy migration in NaCaPO(4)/Mn(2+).

Emission Red Shift and Unusual Band Narrowing of Mn2+ in NaCaPO4 Phosphor

Eu2+-activated LiBaPO4 phosphor was synthesized by conventional solid-state reaction. The photoluminescence excitation and emission spectra, the temperature dependent luminescence intensities (12−450 K), and decay curves of the phosphor were investigated. With the increasing of temperatures, the emission bands of LiBaPO4:Eu2+ show the abnormal blue-shift and the decreasing of emission intensity. The natures of the Eu2+ emission in LiBaPO4, for example, the luminescence quenching temperature, and the activation energy for thermal quenching (ΔE), were reported. The afterglow fluorescence was detected in LiBaPO4:Eu2+ phosphor. Together with the Eu2+ luminescence, Eu3+ ions with the abnormal crystal field were observed. The site-selective excitation in the 5D0 → 7F0 region for Eu3+ ions, emission spectra, and decay curves have been investigated using a pulsed, tunable, and narrowband dye laser to detect the microstructure and crystallographic surrounding of Eu3+,2+ at Ba2+ sites in LiBaPO4. The multiple sites...

Luminescence and Microstructural Features of Eu-Activated LiBaPO4 Phosphor

Thermal and optical properties of Er3+-doped oxyfluorotellurite glasses

Mn4+-doped Y2MgTiO6 phosphors are synthesized by the traditional solid-state method. Powder X-ray diffraction, scanning electron microscope, and energy-dispersive X-ray spectrometer are employed to...

Optical Thermometry Based on Vibration Sidebands in Y2MgTiO6:Mn4+ Double Perovskite

Eu3+ doped red-emitting phosphors of double phosphates Ca9R(PO4)7 (R = Al, Lu) were synthesized by a general high temperature solid-state reaction. The phosphors were characterized by X-ray powder diffraction (XRD). The detailed luminescence properties, e.g., the emission spectra under the excitation of UV light, the photoluminescence excitation spectra and decay lifetimes were reported. The phosphors can be efficiently excited by near UV light to realize an intense red luminescence (613 nm) corresponding to the electric dipole transition 5D0 → 7F2 of Eu3+ ions. The luminescence properties and the potential applications were analyzed. These phosphors were investigated by the site-selective emission spectra and the fluorescence decay curves in the 5D0 → 7F0 region using a pulsed, tunable, narrowband dye laser. It is suggested that Eu3+ ions have three different crystallographic sites doped in Ca9Al(PO4)7, and five sites in Ca9Lu(PO4)7 host. The site assignments of Eu3+ ions in Ca9R(PO4)7 (R = Al, Lu) were discussed on the base of both optical spectroscopy results and structural analysis.

Hyo Jin Seo

Papers

Emission Red Shift and Unusual Band Narrowing of Mn2+ in NaCaPO4 Phosphor

Luminescence and Microstructural Features of Eu-Activated LiBaPO4 Phosphor

Thermal and optical properties of Er3+-doped oxyfluorotellurite glasses

Optical Thermometry Based on Vibration Sidebands in Y2MgTiO6:Mn4+ Double Perovskite

Luminescence properties and site occupations of Eu3+ ions doped in double phosphates Ca9R(PO4)7 (R = Al, Lu)