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Composite quasi-particles with emergent functionalities in spintronic and quantum information science can be realized in correlated materials due to entangled charge, spin, orbital, and lattice degrees of freedom. Here we show that by reducing the lateral dimension of correlated antiferromagnet NiPS 3 flakes to tens of nanometers and thickness to less than ten nanometers, we can switch-off the bulk spin-orbit entangled exciton in the near-infrared (1.47 eV) and activate visible-range (1.8-2.2 eV) transitions. These ultra-sharp lines (<120 μeV at 4.2 K) share the spin-correlated nature of the bulk exciton by displaying a strong linear polarization below Néel temperature. Furthermore, exciton photoluminescence lineshape analysis indicates a polaronic character VIA coupling with at-least 3 phonon modes and a comb-like Stark effect through discretization of charges in each layer. These findings augment the knowledge on the many-body nature of excitonic quasi-particles in correlated antiferromagnets and also establish the nanoscale correlated antiferromagnets as a promising platform for integrated magneto-optic devices.

Current reports of thermal expansion coefficients (TEC) of two-dimensional (2D) materials show large discrepancies that span orders of magnitude. Determining the TEC of any 2D material remains difficult due to approaches involving indirect measurement of samples that are atomically thin and optically transparent. We demonstrate a methodology to address this discrepancy and directly measure TEC of nominally monolayer epitaxial WSe 2 using four-dimensional scanning transmission electron microscopy (4D-STEM). Experimentally, WSe 2 from metal-organic chemical vapor deposition (MOCVD) was heated through a temperature range of 18-564 °C using a barrel-style heating sample holder to observe temperature-induced structural changes without additional alterations or destruction of the sample. By combining 4D-STEM measurements with quantitative structural analysis, the thermal expansion coefficient of nominally monolayer polycrystalline epitaxial 2D WSe 2 was determined to be (3.5 ± 0.9) × 10 -6 K -1 and (5.7 ± 2) × 10 -5 K -1 for the in- and out-of-plane TEC, respectively, and (3.6 ± 0.2) × 10 -5 K -1 for the unit cell volume TEC, in good agreement with historically determined values for bulk crystals.

Understanding the behavior of materials in multi-dimensional architectures composed of atomically thin two-dimensional (2D) materials and three-dimensional (3D) materials has become mandatory for progress in materials preparation via various epitaxy techniques, such as van der Waals and remote epitaxy methods. We investigated the growth behavior of ZnO on monolayer MoS 2 as a model system to study the growth of a 3D material on a 2D material, which is beyond the scope of remote and van der Waals epitaxy. The study revealed column-to-column alignment and inversion of crystallinity, which can be explained by combinatorial epitaxy, grain alignment across an atomically sharp interface, and a compliant substrate. The growth study enabled the formation of a ZnO/MoS 2 heterostructure with type-I band alignment. Our findings will have a scientific impact on realizing 2D/3D heterostructures for practical device applications.

Confining materials to two-dimensional forms changes the behaviour of the electrons and enables the creation of new devices. However, most materials are challenging to produce as uniform, thin crystals. Here we present a synthesis approach where thin crystals are grown in a nanoscale mould defined by atomically flat van der Waals (vdW) materials. By heating and compressing bismuth in a vdW mould made of hexagonal boron nitride, we grow ultraflat bismuth crystals less than 10 nm thick. Due to quantum confinement, the bismuth bulk states are gapped, isolating intrinsic Rashba surface states for transport studies. The vdW-moulded bismuth shows exceptional electronic transport, enabling the observation of Shubnikov-de Haas quantum oscillations originating from the (111) surface state Landau levels. By measuring the gate-dependent magnetoresistance, we observe multi-carrier quantum oscillations and Landau level splitting, with features originating from both the top and bottom surfaces. Our vdW mould growth technique establishes a platform for electronic studies and control of bismuth's Rashba surface states and topological boundary modes 1-3 . Beyond bismuth, the vdW-moulding approach provides a low-cost way to synthesize ultrathin crystals and directly integrate them into a vdW heterostructure., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

The fine-tuning of topologically protected states in quantum materials holds great promise for novel electronic devices. However, there are limited methods that allow for the controlled and efficient modulation of the crystal lattice while simultaneously monitoring the changes in the electronic structure within a single sample. Here, we apply significant and controllable strain to high-quality HfTe 5 samples and perform electrical transport measurements to reveal the topological phase transition from a weak topological insulator phase to a strong topological insulator phase. After applying high strain to HfTe 5 and converting it into a strong topological insulator, we found that the resistivity of the sample increased by 190,500% and that the electronic transport was dominated by the topological surface states at cryogenic temperatures. Our results demonstrate the suitability of HfTe 5 as a material for engineering topological properties, with the potential to generalize this approach to study topological phase transitions in van der Waals materials and heterostructures., (© 2024. The Author(s).)

Thermoelectric materials with high electrical conductivity and low thermal conductivity (e.g., Bi 2 Te 3 ) can efficiently convert waste heat into electricity; however, in spite of favorable theoretical predictions, individual Bi 2 Te 3 nanostructures tend to perform less efficiently than bulk Bi 2 Te 3 . We report a greater-than-order-of-magnitude enhancement in the thermoelectric properties of suspended Bi 2 Te 3 nanoribbons, coated in situ to form a Bi 2 Te 3 /F 4 -TCNQ core-shell nanoribbon without oxidizing the core-shell interface. The shell serves as an oxidation barrier but also directly functions as a strong electron acceptor and p-type carrier donor, switching the majority carriers from a dominant n-type carrier concentration (∼10 21 cm -3 ) to a dominant p-type carrier concentration (∼10 20 cm -3 ). Compared to uncoated Bi 2 Te 3 nanoribbons, our Bi 2 Te 3 /F 4 -TCNQ core-shell nanoribbon demonstrates an effective chemical potential dramatically shifted toward the valence band (by 300-640 meV), robustly increased Seebeck coefficient (∼6× at 250 K), and improved thermoelectric performance (10-20× at 250 K).

Interlayer excitons (IXs) formed at the interface of van der Waals materials possess various novel properties. In parallel development, strain engineering has emerged as an effective means for creating 2D quantum emitters. Exploring the intersection of these two exciting areas, we use MoS 2 /WSe 2 heterostructure as a model system and demonstrate how strain, defects, and layering can be utilized to create defect-bound IXs capable of bright, robust, and tunable quantum light emission in the technologically important near-infrared spectral range. Our work presents defect-bound IXs as a promising platform for pushing the performance of 2D quantum emitters beyond their current limitations.

Quantum light emitters capable of generating single photons with circular polarization and non-classical statistics could enable non-reciprocal single-photon devices and deterministic spin-photon interfaces for quantum networks. To date, the emission of such chiral quantum light relies on the application of intense external magnetic fields, electrical/optical injection of spin-polarized carriers/excitons or coupling with complex photonic metastructures. Here we report the creation of free-space chiral quantum light emitters via the nanoindentation of monolayer WSe 2 /NiPS 3 heterostructures at zero external magnetic field. These quantum light emitters emit with a high degree of circular polarization (0.89) and single-photon purity (95%), independent of pump laser polarization. Scanning diamond nitrogen-vacancy microscopy and temperature-dependent magneto-photoluminescence studies reveal that the chiral quantum light emission arises from magnetic proximity interactions between localized excitons in the WSe 2 monolayer and the out-of-plane magnetization of defects in the antiferromagnetic order of NiPS 3 , both of which are co-localized by strain fields associated with the nanoscale indentations., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

Defect engineering of van der Waals semiconductors has been demonstrated as an effective approach to manipulate the structural and functional characteristics toward dynamic device controls, yet correlations between physical properties with defect evolution remain underexplored. Using proton irradiation, we observe an enhanced exciton-to-trion conversion of the atomically thin WS 2 . The altered excitonic states are closely correlated with nanopore induced atomic displacement, W nanoclusters, and zigzag edge terminations, verified by scanning transmission electron microscopy, photoluminescence, and Raman spectroscopy. Density functional theory calculation suggests that nanopores facilitate formation of in-gap states that act as sinks for free electrons to couple with excitons. The ion energy loss simulation predicts a dominating electron ionization effect upon proton irradiation, providing further evidence on band perturbations and nanopore formation without destroying the overall crystallinity. This study provides a route in tuning the excitonic properties of van der Waals semiconductors using an irradiation-based defect engineering approach.

Atomically thin transition metal dichalcogenides (TMDs), like MoS 2 with high carrier mobilities and tunable electron dispersions, are unique active material candidates for next generation opto-electronic devices. Previous studies on ion irradiation show great potential applications when applied to two-dimensional (2D) materials, yet have been limited to micron size exfoliated flakes or smaller. To demonstrate the scalability of this method for industrial applications, we report the application of relatively low power (50 keV) 4 He + ion irradiation towards tuning the optoelectronic properties of an epitaxially grown continuous film of MoS 2 at the wafer scale, and demonstrate that precise manipulation of atomistic defects can be achieved in TMD films using ion implanters. The effect of 4 He + ion fluence on the PL and Raman signatures of the irradiated film provides new insights into the type and concentration of defects formed in the MoS 2 lattice, which are quantified through ion beam analysis. PL and Raman spectroscopy indicate that point defects are generated without causing disruption to the underlying lattice structure of the 2D films and hence, this technique can prove to be an effective way to achieve defect-mediated control over the opto-electronic properties of MoS 2 and other 2D materials., (Creative Commons Attribution license.)

Actinide materials have various applications that range from nuclear energy to quantum computing. Most current efforts have focused on bulk actinide materials. Tuning functional properties by using strain engineering in epitaxial thin films is largely lacking. Using uranium dioxide (UO 2 ) as a model system, in this work, the authors explore strain engineering in actinide epitaxial thin films and investigate the origin of induced ferromagnetism in an antiferromagnet UO 2 . It is found that UO 2+ x thin films are hypostoichiometric (x<0) with in-plane tensile strain, while they are hyperstoichiometric (x>0) with in-plane compressive strain. Different from strain engineering in non-actinide oxide thin films, the epitaxial strain in UO 2 is accommodated by point defects such as vacancies and interstitials due to the low formation energy. Both epitaxial strain and strain relaxation induced point defects such as oxygen/uranium vacancies and oxygen/uranium interstitials can distort magnetic structure and result in magnetic moments. This work reveals the correlation among strain, point defects and ferromagnetism in strain engineered UO 2+ x thin films and the results offer new opportunities to understand the influence of coupled order parameters on the emergent properties of many other actinide thin films., (© 2022 The Authors. Advanced Science published by Wiley-VCH GmbH.)

This paper reports on the manufacturing of complex three-dimensional Si/C structures via a chemical vapor deposition method. The structure and properties of the grown materials were characterized using various techniques including scanning electron microscopy, aberration-corrected transmission electron microscopy, confocal Raman spectroscopy, and X-ray photoelectron spectroscopy. The spectroscopy results revealed that the grown materials were composed of micro/nanostructures with various compositions and dimensions. These included two-dimensional silicon carbide (SiC), cubic silicon, and various SiC polytypes. The coexistence of these phases at the nano-level and their interfaces can benefit several Si/C-based applications ranging from ceramics and structural applications to power electronics, aerospace, and high-temperature applications. With an average density of 7 mg/cm 3 , the grown materials can be considered ultralightweight, as they are three orders of magnitude lighter than bulk Si/C materials. This study aims to impact how ceramic materials are manufactured, which may lead to the design of new carbide materials or Si/C-based lightweight structures with additional functionalities and desired properties.

Using four-dimensional scanning transmission electron microscopy, we demonstrate a method to visualize grains and grain boundaries in WSe 2 grown by metal organic chemical vapor deposition (MOCVD) directly onto silicon dioxide. Despite the chemical purity and uniform thickness and texture of the MOCVD-grown WSe 2 , we observe a high density of small grains that corresponds with the overall selenium deficiency we measure through ion beam analysis. Moreover, reconstruction of grain information permits the creation of orientation maps that demonstrate the nucleation mechanism for new layers-triangular domains with the same orientation as the layer underneath induces a tensile strain increasing the lattice parameter at these sites.

Quantum emitters (QEs) in two-dimensional transition metal dichalcogenides (2D TMDCs) have advanced to the forefront of quantum communication and transduction research. To date, QEs capable of operating in O-C telecommunication bands have not been demonstrated in TMDCs. Here we report site-controlled creation of telecom QEs emitting over the 1080 to 1550 nm telecommunication wavelength range via coupling of 2D molybdenum ditelluride (MoTe 2 ) to strain inducing nano-pillar arrays. Hanbury Brown and Twiss experiments conducted at 10 K reveal clear photon antibunching with 90% single-photon purity. The photon antibunching can be observed up to liquid nitrogen temperature (77 K). Polarization analysis further reveals that while some QEs display cross-linearly polarized doublets with ~1 meV splitting resulting from the strain induced anisotropic exchange interaction, valley degeneracy is preserved in other QEs. Valley Zeeman splitting as well as restoring of valley symmetry in cross-polarized doublets are observed under 8 T magnetic field., (© 2021. This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.)

Robust atomic-to-meso-scale chirality is now observed in the one-dimensional form of tellurium. This enables a large and counter-intuitive circular-polarization dependent second harmonic generation response above 0.2 which is not present in two-dimensional tellurium. Orientation variations in 1D tellurium nanowires obtained by four-dimensional scanning transmission electron microscopy (4D-STEM) and their correlation with unconventional non-linear optical properties by second harmonic generation circular dichroism (SHG-CD) uncovers an unexpected circular-polarization dependent SHG response from 1D nanowire bundles - an order-of-magnitude higher than in single-crystal two-dimensional tellurium structures - suggesting the atomic- and meso-scale crystalline structure of the 1D material possesses an inherent chirality not present in its 2D form; and which is strong enough to manifest even in the aggregate non-linear optical (NLO) properties of aggregates.

Antimony selenide (Sb 2 Se 3 ) is a material widely used in photodetectors and relatively new as a possible material for thermoelectric applications. Taking advantage of the new properties after nanoscale fabrication, this material shows great potential for the development of efficient low temperature thermoelectric devices. Here we study the synthesis, the crystal properties and the thermal and thermoelectric transport response of Sb 2 Se 3 hexagonal nanotubes (HNT) in the temperature range between 120 and 370 K. HNT have a moderate electrical conductivity ∼10 2 S m -1 while maintaining a reasonable Seebeck coefficient ∼430 μV K -1 at 370 K. The electrical conductivity in Sb 2 Se 3 HNT is about 5 orders of magnitude larger and its thermal conductivity one half of what is found in bulk. Moreover, the calculated figure of merit (ZT) at room temperature is the largest value reported in antimony selenide 1D structures.

A new microwave-enhanced synthesis method for the production of tellurium nanostructures is reported-with control over products from the 1D regime (sub-5 nm diameter nanowires), to nanoribbons, to the 2D tellurene regime-along with a new methodology for local statistical quantification of the crystallographic parameters of these materials at the nanometer scale. Using a direct electron detector and image-corrected microscope, large and robust 4D scanning transmission electron microscopy datasets for accurate structural analysis are obtained. These datasets allow the adaptation of quantitative techniques originally developed for X-ray diffraction (XRD) refinement analyses to transmission electron microscopy, enabling the first demonstration of sub-picometer accuracy lattice parameter extraction while also obtaining both the size of the coherent crystallite domains and the nanostrain, which is observed to decrease as nanowires transition to tellurene. This new local analysis is commensurate with global powder XRD results, indicating the robustness of both the new synthesis approach and new structural analysis methodology for future scalable production of 2D tellurene and characterization of nanomaterials., (© 2020 Wiley-VCH GmbH.)

Isotopes of an element have the same electron number but differ in neutron number and atomic mass. However, due to the thickness-dependent properties in MX 2 (M = Mo, W; X = S, Se, Te) transition metal dichalcogenides (TMDs), the isotopic effect in atomically thin TMDs still remains unclear especially for phonon-assisted indirect excitonic transitions. Here, we report the first observation of the isotope effect on the electronic and vibrational properties of a TMD material, using naturally abundant NA W NA Se 2 and isotopically pure 186 W 80 Se 2 bilayer single crystals over a temperature range of 4.4-300 K. We demonstrate a higher optical band gap energy in  186 W 80 Se 2 than in NA W NA Se 2 (3.9 ± 0.7 meV from 4.41 to 300 K), which is surprising as isotopes are neutral impurities. Phonon energies decrease in the isotopically pure crystal due to the atomic mass dependence of harmonic oscillations, with correspondingly longer E 2g and A 2 1g phonon lifetimes than in the naturally abundant sample. The change in electronic band gap renormalization energy is postulated as being the dominant mechanism responsible for the change in optical emission spectra.

This review is concerned with the leading methods of bottom-up material preparation for thermal-to-electrical energy interconversion. The advantages, capabilities, and challenges from a material synthesis perspective are surveyed and the methods are discussed with respect to their potential for improvement (or possibly deterioration) of application-relevant transport properties. Solution chemistry-based synthesis approaches are re-assessed from the perspective of thermoelectric applications based on reported procedures for nanowire, quantum dot, mesoporous, hydro/solvothermal, and microwave-assisted syntheses as these techniques can effectively be exploited for industrial mass production. In terms of energy conversion efficiency, the benefit of self-assembly can occur from three paths: suppressing thermal conductivity, increasing thermopower, and boosting electrical conductivity. An ideal thermoelectric material gains from all three improvements simultaneously. Most bottom-up materials have been shown to exhibit very low values of thermal conductivity compared to their top-down (solid-state) counterparts, although the main challenge lies in improving their poor electrical properties. Recent developments in the field discussed in this review reveal that the traditional view of bottom-up thermoelectrics as inferior materials suffering from poor performance is not appropriate. Thermopower enhancement due to size and energy filtering effects, electrical conductivity enhancement, and thermal conductivity reduction mechanisms inherent in bottom-up nanoscale self-assembly syntheses are indicative of the impact that these techniques will play in future thermoelectric applications.

Highly crystalline indium tin oxide (ITO) nanowires were grown via a vapor-liquid-solid method, with thermal tolerance up to ∼1300 °C. We report the electric and thermoelectric properties of the ITO nanowires before and after heat treatments and draw conclusions about their applicability as thermoelectric building blocks in nanodevices that can operate in high temperature conditions. The Seebeck coefficient and the thermal and electrical conductivities were measured in each individual nanowire by means of specialized micro-bridge thermometry devices. Measured data was analyzed and explained in terms of changes in charge carrier density, impurities and vacancies due to the thermal treatments.

Tin selenide (SnSe) has been the subject of great attention in the last years due to its highly efficient thermoelectricity and its possibilities as a green material, free of Pb and Te. Here, we report for the first time a thermoelectricity and transport study of individual SnSe micro- and nano-wires with diameters in the range between 130 nm and 1.15 μm. X-ray diffraction and transmission electron microscopy analyses confirm an orthorhombic SnSe structure with Pnma (62) symmetry group and 1:1 Sn:Se atomic ratio. Electrical and thermal conductivity and the Seebeck coefficient were measured in each individual nanowire using a specialized suspended microdevice in the 150-370 K temperature range, yielding a thermal conductivity of 0.55 Wm -1  K -1 at room temperature and ZT ~ 0.156 at 370 K for the 130 nm diameter nanowire. The measured properties were correlated with electronic information obtained by model simulations and with phonon scattering analysis. The results confirm these structures as promising building blocks to develop efficient temperature sensors, refrigerators and thermoelectric energy converters. The thermoelectric response of the nanowires is compared with recent reports on crystalline, polycrystalline and layered bulk structures.

Experimental investigations of solid materials at elevated temperatures rely on the optimized thermal design of the measurement system, as radiation becomes a predominant source of heat loss which can lead to large uncertainty in measured temperature and related physical properties of a test sample. Advancements in surface temperature measurements have reduced thermal losses arising from the cold-finger effect using axially inserted thermocouples and from radiation using shields or other thermal guards. The leading technology for temperature sensing at temperatures up to ∼900 °C makes use of these design features for measuring thermopower, yet uncertainty analysis estimation of this technique is not known. This work makes use of finite element modeling to determine spatial temperature distributions to obtain the upper limit of confidence expected for the axially inserted thermocouple approach when a heated radiation shield is incorporated into the design. Using an axially inserted thermocouple to measure the sample surface temperature, the temperature variations across the sample hot and cold surfaces at 900 °C for a temperature drop of 0, 5, and 10 °C are calculated to be as low as 0.02, 0.21, and 0.41 °C, respectively, when a heated radiation shield is employed. Uniform temperature distribution on the thermocouple cross-wire geometry indicates that the axial thermocouple measurement design is indeed effective for suppressing the cold-finger effect. Using a heated radiation shield is found to significantly reduce the temperature gradient across the thermocouples.

The presence of oxygen vacancy sites fundamentally affects physical and chemical properties of materials. In this study, a dipole-containing interaction between poly(diallyldimethylammonium chloride) PDDA and α-MoO 3 is found to enable high-concentrations of surface oxygen vacancies. Thermal annealing under Ar resulted in negligible reduction of MoO 3 to MoO 3- x with x = 0.03 at 600 °C. In contrast, we show that the thermochemical reaction with PDDA polyelectrolyte under Ar can significantly reduce MoO 3 to MoO 3- x with x = 0.36 (MoO 2.64 ) at 600 °C. Thermal annealing under H 2 gas enhanced the substoichiometry of MoO 3- x from x = 0.62 to 0.98 by using PDDA at the same conditions. Density functional theory calculations, supported by experimental analysis, suggest that the vacancy sites are created through absorption of terminal site oxygen (O t ) upon decomposition of the N-C bond in the pentagonal ring of PDDA during the thermal treatment. O t atoms are absorbed as ionic O - and neutral O 2- , creating Mo 5+ -v O · and Mo 4+ -v O ·· vacancy bipolarons and polarons, respectively. X-ray photoemission spectroscopy peak analysis indicates the ratio of charged to neutral molybdenum ions in the PDDA-processed samples increased from Mo 4+ /Mo 6+ = 1.0 and Mo 5+ /Mo 6+ = 3.3 when reduced at 400 °C to Mo 4+ /Mo 6+ = 3.7 and Mo 5+ /Mo 6+ = 2.6 when reduced at 600 °C. This is consistent with our ab initio calculation where the Mo 4+ -v O ·· formation energy is 0.22 eV higher than that for Mo 5+ -v O · in the bulk of the material and 0.02 eV higher on the surface. This study reveals a new paradigm for effective enhancement of surface oxygen vacancy concentrations essential for a variety of technologies including advanced energy conversion applications such as electrochemical energy storage, catalysis, and low-temperature thermochemical water splitting.

Transition metal dichalcogenides (TMDs) are particularly sensitive to mechanical strain because they are capable of experiencing high atomic displacements without nucleating defects to release excess energy. Being promising for photonic applications, it has been shown that as certain phases of layered TMDs MX 2 (M = Mo or W; X = S, Se, or Te) are scaled to a thickness of one monolayer, the photoluminescence response is dramatically enhanced due to the emergence of a direct electronic band gap compared with their multilayer or bulk counterparts, which typically exhibit indirect band gaps. Recently, mechanical strain has also been predicted to enable direct excitonic recombination in these materials, in which large changes in the photoluminescence response will occur during an indirect-to-direct band gap transition brought on by elastic tensile strain. Here, we demonstrate an enhancement of 2 orders of magnitude in the photoluminescence emission intensity in uniaxially strained single crystalline WSe 2 bilayers. Through a theoretical model that includes experimentally relevant system conditions, we determine this amplification to arise from a significant increase in direct excitonic recombination. Adding confidence to the high levels of elastic strain achieved in this report, we observe strain-independent, mode-dependent Grüneisen parameters over the entire range of tensile strain (1-3.59%), which were obtained as 1.149 ± 0.027, 0.307 ± 0.061, and 0.357 ± 0.103 for the E 2g , A 1g , and A 2 1g optical phonon modes, respectively. These results can inform the predictive strain-engineered design of other atomically thin indirect semiconductors, in which a decrease in out-of-plane bonding strength may lead to an increase in the strength of strain-coupled optoelectronic effects.

Hollow spherical structures of ternary bismuth molybdenum oxide doped with samarium (Bi 2-x Sm x MoO 6 ) were successfully synthesized via development of a Pluronic P123 (PEO 20 -PPO 70 -PEO 20 )-assisted solvothermal technique. Density functional theory calculations have been performed to improve our understanding of the effects of Sm doping on the electronic band structure, density of states, and band gap of the material. The calculations for 0 ≤ x ≤ 0.3 revealed a considerably flattened conduction band minimum near the Γ point, suggesting that the material can be considered to possess a quasi-direct band gap. In contrast, for x = 0.5, the conduction band minimum is deflected toward the U point, making it a distinctly indirect band gap material. The effects of a hollow structure as well as Sm substitution on the absorbance and fluorescence properties of the materials produced increased emission intensities at low Sm concentrations (x = 0.1 and 0.3), with x = 0.1 displaying a peak photoluminescence intensity 13.2 times higher than for the undoped bulk sample. Subsequent increases in the Sm concentration resulted in quenching of the emission intensity, indicative of the onset of a quasi-direct-to-indirect electronic band transition. These results indicate that both mesoscale structuring and Sm doping will be promising routes for tuning optoelectronic properties for future applications such as catalysis and photocatalysis.

The suspended micro-thermometry measurement technique is one of the most prominent methods for probing the in-plane thermal conductance of low dimensional materials, where a suspended microdevice containing two built-in platinum resistors that serve as both heater and thermometer is used to measure the temperature and heat flow across a sample. The presence of temperature fluctuations in the sample chamber and background thermal conductance through the device, residual gases, and radiation are dominant sources of error when the sample thermal conductance is comparable to or smaller than the background thermal conductance, on the order of 300 pW/K at room temperature. In this work, we present a high resolution thermal conductance measurement scheme in which a bipolar direct current reversal technique is adopted to replace the lock-in technique. We have demonstrated temperature resolution of 1.0-2.6 mK and thermal conductance resolution of 1.7-26 pW/K over a temperature range of 30-375 K. The background thermal conductance of the suspended microdevice is determined accurately by our method and allows for straightforward isolation of this parasitic signal. This simple and high-throughput measurement technique yields an order of magnitude improvement in resolution over similarly configured lock-in amplifier techniques, allowing for more accurate investigation of fundamental phonon transport mechanisms in individual nanomaterials.

The spin-helical Dirac fermion topological surface states in a topological insulator nanowire or nanoribbon promise novel topological devices and exotic physics such as Majorana fermions. Here, we report local and non-local transport measurements in Bi2Te3 topological insulator nanoribbons that exhibit quasi-ballistic transport over ∼2 μm. The conductance versus axial magnetic flux Φ exhibits Aharonov-Bohm oscillations with maxima occurring alternately at half-integer or integer flux quanta (Φ0 = h/e, where h is Planck's constant and e is the electron charge) depending periodically on the gate-tuned Fermi wavevector (kF) with period 2π/C (where C is the nanoribbon circumference). The conductance versus gate voltage also exhibits kF-periodic oscillations, anti-correlated between Φ = 0 and Φ0/2. These oscillations enable us to probe the Bi2Te3 band structure, and are consistent with the circumferentially quantized topological surface states forming a series of one-dimensional subbands, which undergo periodic magnetic field-induced topological transitions with the disappearance/appearance of the gapless Dirac point with a one-dimensional spin helical mode.

Suspended microdevices are employed to measure the in-plane electrical conductivity, thermal conductivity, and Seebeck coefficient of suspended poly(3,4-ethylenedioxythiophene) (PEDOT) thin films. The measured thermal conductivity is higher than previously reported for PEDOT and generally increases with the electrical conductivity. The increase exceeds that predicted by the Wiedemann-Franz law for metals and can be explained by significant electronic thermal transport in PEDOT., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Transport due to spin-helical massless Dirac fermion surface state is of paramount importance to realize various new physical phenomena in topological insulators, ranging from quantum anomalous Hall effect to Majorana fermions. However, one of the most important hallmarks of topological surface states, the Dirac linear band dispersion, has been difficult to reveal directly in transport measurements. Here we report experiments on Bi2Te3 nanoribbon ambipolar field effect devices on high-κ SrTiO3 substrates, where we achieve a gate-tuned bulk metal-insulator transition and the topological transport regime with substantial surface state conduction. In this regime, we report two unambiguous transport evidences for gate-tunable Dirac fermions through π Berry's phase in Shubnikov-de Haas oscillations and effective mass proportional to the Fermi momentum, indicating linear energy-momentum dispersion. We also measure a gate-tunable weak anti-localization (WAL) with 2 coherent conduction channels (indicating 2 decoupled surfaces) near the charge neutrality point, and a transition to weak localization (indicating a collapse of the Berry's phase) when the Fermi energy approaches the bulk conduction band. The gate-tunable Dirac fermion topological surface states pave the way towards a variety of topological electronic devices.

Polymers are usually considered thermal insulators, because the amorphous arrangement of the molecular chains reduces the mean free path of heat-conducting phonons. The most common method to increase thermal conductivity is to draw polymeric fibres, which increases chain alignment and crystallinity, but creates a material that currently has limited thermal applications. Here we show that pure polythiophene nanofibres can have a thermal conductivity up to ∼ 4.4 W m(-1) K(-1) (more than 20 times higher than the bulk polymer value) while remaining amorphous. This enhancement results from significant molecular chain orientation along the fibre axis that is obtained during electropolymerization using nanoscale templates. Thermal conductivity data suggest that, unlike in drawn crystalline fibres, in our fibres the dominant phonon-scattering process at room temperature is still related to structural disorder. Using vertically aligned arrays of nanofibres, we demonstrate effective heat transfer at critical contacts in electronic devices operating under high-power conditions at 200 °C over numerous cycles.

A microdevice was used to measure the in-plane thermoelectric properties of suspended bismuth telluride nanoplates from 9 to 25 nm thick. The results reveal a suppressed Seebeck coefficient together with a general trend of decreasing electrical conductivity and thermal conductivity with decreasing thickness. While the electrical conductivity of the nanoplates is still within the range reported for bulk Bi2Te3, the total thermal conductivity for nanoplates less than 20 nm thick is well below the reported bulk range. These results are explained by the presence of surface band bending and diffuse surface scattering of electrons and phonons in the nanoplates, where pronounced n-type surface band bending can yield suppressed and even negative Seebeck coefficient in unintentionally p-type doped nanoplates.

An increasingly used technique for measuring the thermal conductance of a nanowire is based on a suspended micro-device with built-in resistance thermometers. In the past, the technique has been limited to samples with thermal conductance larger than 1 × 10(-9) W/K because of temperature fluctuations in the sample environment and the presence of background heat transfer through residual gas molecules and radiation between the two thermometers. In addition, parasitic heat loss from the long supporting beams and asymmetry in the fabricated device results in two additional errors, which have been ignored in previous use of this method. To address these issues, we present a comprehensive measurement approach, where the device asymmetry is determined by conducting thermal measurements with two opposite heat flow directions along the nanowire, the background heat transfer is eliminated by measuring the differential heat transfer signal between the nanowire device and a reference device without a nanowire sample, and the parasitic heat loss from the supporting beams is obtained by measuring the average temperature rise of one of the beams. This technique is demonstrated on a nanofiber sample with a thermal conductance of 3.7 × 10(-10) W/K, against a background conductance of 8.2 × 10(-10) W/K at 320 K temperature. The results reveal the need to reduce the background thermal conductance in order to employ the micro-device to measure a nanowire sample with the thermal conductance less than 1 × 10(-10) W/K.

The thermal conductivity of suspended few-layer hexagonal boron nitride (h-BN) was measured using a microbridge device with built-in resistance thermometers. Based on the measured thermal resistance values of 11-12 atomic layer h-BN samples with suspended lengths ranging between 3 and 7.5 μm, the room-temperature thermal conductivity of a 11-layer sample was found to be about 360 W m(-1) K(-1), approaching the basal plane value reported for bulk h-BN. The presence of a polymer residue layer on the sample surface was found to decrease the thermal conductivity of a 5-layer h-BN sample to be about 250 W m(-1) K(-1) at 300 K. Thermal conductivities for both the 5-layer and the 11-layer samples are suppressed at low temperatures, suggesting increasing scattering of low frequency phonons in thin h-BN samples by polymer residue.

At a very low solid concentration of 0.45 ± 0.09 vol %, the room-temperature thermal conductivity (κ(GF)) of freestanding graphene-based foams (GF), comprised of few-layer graphene (FLG) and ultrathin graphite (UG) synthesized through the use of methane chemical vapor deposition on reticulated nickel foams, was increased from 0.26 to 1.7 W m(-1) K(-1) after the etchant for the sacrificial nickel support was changed from an aggressive hydrochloric acid solution to a slow ammonium persulfate etchant. In addition, κ(GF) showed a quadratic dependence on temperature between 11 and 75 K and peaked at about 150 K, where the solid thermal conductivity (κ(G)) of the FLG and UG constituents reached about 1600 W m(-1) K(-1), revealing the benefit of eliminating internal contact thermal resistance in the continuous GF structure.

We report the use of free-standing, lightweight, and highly conductive ultrathin graphite foam (UGF), loaded with lithium iron phosphate (LFP), as a cathode in a lithium ion battery. At a high charge/discharge current density of 1280 mA g(-1), the specific capacity of the LFP loaded on UGF was 70 mAh g(-1), while LFP loaded on Al foil failed. Accounting for the total mass of the electrode, the maximum specific capacity of the UGF/LFP cathode was 23% higher than that of the Al/LFP cathode and 170% higher than that of the Ni-foam/LFP cathode. Using UGF, both a higher rate capability and specific capacity can be achieved simultaneously, owing to its conductive (∼1.3 × 10(5) S m(-1) at room temperature) and three-dimensional lightweight (∼9.5 mg cm(-3)) graphitic structure. Meanwhile, UGF presents excellent electrochemical stability comparing to that of Al and Ni foils, which are generally used as conductive substrates in lithium ion batteries. Moreover, preparation of the UGF electrode was facile, cost-effective, and compatible with various electrochemically active materials.

The thermal conductivity (κ) of two bilayer graphene samples each suspended between two microresistance thermometers was measured to be 620 ± 80 and 560 ± 70 W m(-1) K(-1) at room temperature and exhibits a κ ∝ T(1.5) behavior at temperatures (T) between 50 and 125 K. The lower κ than that calculated for suspended graphene along with the temperature dependence is attributed to scattering of phonons in the bilayer graphene by a residual polymeric layer that was clearly observed by transmission electron microscopy.

The reported thermal conductivity (kappa) of suspended graphene, 3000 to 5000 watts per meter per kelvin, exceeds that of diamond and graphite. Thus, graphene can be useful in solving heat dissipation problems such as those in nanoelectronics. However, contact with a substrate could affect the thermal transport properties of graphene. Here, we show experimentally that kappa of monolayer graphene exfoliated on a silicon dioxide support is still as high as about 600 watts per meter per kelvin near room temperature, exceeding those of metals such as copper. It is lower than that of suspended graphene because of phonons leaking across the graphene-support interface and strong interface-scattering of flexural modes, which make a large contribution to kappa in suspended graphene according to a theoretical calculation.

A focused laser beam is used to heat individual single-walled carbon nanotube bundles bridging two suspended microthermometers. By measurement of the temperature rise of the two thermometers, the optical absorption of 7.4-10.3 nm diameter bundles is found to be between 0.03 and 0.44% of the incident photons in the 0.4 microm diameter laser spot. The thermal conductance of the bundle is obtained with the additional measurement of the temperature rise of the nanotubes in the laser spot from shifts in the Raman G band frequency. According to the nanotube bundle diameter determined by transmission electron microscopy, the thermal conductivity is obtained.

The Seebeck coefficient, electrical conductivity, and thermal conductivity of individual chromium disilicide nanowires were characterized using a suspended microdevice and correlated with the crystal structure and growth direction obtained by transmission electron microscopy on the same nanowires. The obtained thermoelectric figure of merit of the nanowires was comparable to the bulk values. We show that combined Seebeck coefficient and electrical conductivity measurements provide an effective approach to probing the Fermi Level, carrier concentration and mobility in nanowires.

Measuring in-plane thermoelectric properties of submicron thin films has remained a challenging task. Here we report a method based on a suspended microdevice for four-probe measurements of the Seebeck coefficient, thermal conductivity, electrical conductivity, and thermoelectric figure of merit of patterned indium arsenide (InAs) nanofilms assembled on the microdevice. The contact thermal resistance and intrinsic thermal resistance of the 40 nm thick InAs nanofilm sample were measured by using the nanofilm itself as a differential thermocouple to determine the temperature drops at the contacts. The microdevice was also used to measure a 190 nm thick silicon dioxide (SiO(2)) film and the results were compared with those reported in the literature. A through-substrate hole under the suspended microdevice allows for transmission electron microscopy characterization of the nanofilm sample assembled on the device. This capability enables one to correlate the measured thermoelectric properties with the crystal structures of the nanofilm.