The cross-plane thermal conductivity of thin films of WSe2 grown from alternating W and Se layers is as small as 0.05 watts per meter per degree kelvin at room temperature, 30 times smaller than the c-axis thermal conductivity of single-crystal WSe2 and a factor of 6 smaller than the predicted minimum thermal conductivity for this material. We attribute the ultralow thermal conductivity of these disordered, layered crystals to the localization of lattice vibrations induced by the random stacking of two-dimensional crystalline WSe2 sheets. Disordering of the layered structure by ion bombardment increases the thermal conductivity.

We describe a simple approach for rejecting unwanted scattered light in two types of time-resolved pump-probe measurements, time-domain thermoreflectance (TDTR) and time-resolved incoherent anti-Stokes Raman scattering (TRIARS). Sharp edged optical filters are used to create spectrally distinct pump and probe beams from the broad spectral output of a femtosecond Ti:sapphire laser oscillator. For TDTR, the diffusely scattered pump light is then blocked by a third optical filter. For TRIARS, depolarized scattering created by the pump is shifted in frequency by approximately 250 cm(-1) relative to the polarized scattering created by the probe; therefore, spectral features created by the pump and probe scattering can be easily distinguished.

We report the room-temperature, cross-plane thermal conductivities, and longitudinal speeds of sound of multilayer films [(TiTe2)(3)(Bi2Te3)(x)(TiTe2)(3)(Sb2Te3)(y)](i) (x=1-5, y=1-5) and misfit-layer dichalcogenide films [(PbSe)(m)(TSe2)(n)](i) (T=W or Mo, m=1-5, and n=1-5) synthesized by the modulated elemental reactants method. The thermal conductivities of these nanoscale layered materials fall below the predicted minimum thermal conductivity of the component compounds: two times lower than the minimum thermal conductivity of Bi2Te3 for multilayer [(TiTe2)(3)(Bi2Te3)(x)(TiTe2)(3)(Sb2Te3)(y)](i) films and five to six times lower than the minimum thermal conductivity of PbSe for misfit-layer dichalcogenides [(PbSe)(m)(TSe2)(n)](i). We attribute the low thermal conductivities to the anisotropic bonding of the layered crystals and orientational disorder in the stacking of layered crystals along the direction perpendicular to the surface

We investigate the lower limit to the lattice thermal conductivity of Bi2Te3 and related materials using thin films synthesized by the method of elemental reactants. The thermal conductivities of single layer films of (Bi0.5Sb0.5)2Te3 and multilayer films of (Bi2Te3)m(TiTe2)n and [(BixSb1−x)2Te3]m(TiTe2)n are measured by time-domain thermoreflectance; the thermal conductivity data are compared to our prior work on nanocrystalline Bi2Te3 and a Debye–Callaway model of heat transport by acoustic phonons. The homogeneous nanocrystalline films have average grain sizes 30<d<100 nm as measured by the width of the (003) x-ray diffraction peak. Multilayer films incorporating turbostratic TiTe2 enable studies of the effective thermal conductivity of Bi2Te3 layers as thin as 2 nm. In the limit of small grain size or layer thickness, the thermal conductivity of Bi2Te3 approaches the predicted minimum thermal conductivity of 0.31 W/m K. The dependence of the thermal conductivity on grain size is in good agreement with our Debye–Callaway model. The use of alloy (Bi,Sb)2Te3 layers further reduces the thermal conductivity of the nanoscale layers to as low as 0.20 W/m K.

The response of CR-39 track detectors to neutrons has been characterized and used to measure neutron yields from implosions of DD-and DT-filled targets at the OMEGA laser facility ͓T. R. Boehly et al., Opt. Commun. 133, 495 ͑1997͔͒, and the scaling of neutron fluence with R ͑the target-to-detector distance͒ has been used to characterize the fluence of backscattered neutrons in the target chamber. A Monte-Carlo code was developed to predict the CR-39 efficiency for detecting DD neutrons, and it agrees well with the measurements. Neutron detection efficiencies of ͑1.1 Ϯ0.2)ϫ10 Ϫ4 and (6.0Ϯ0.7)ϫ10 Ϫ5 for the DD and DT cases, respectively, were determined for standard CR-39 etch conditions. In OMEGA experiments with both DD and DT targets, the neutron fluence was observed to decrease as R Ϫ2 up to about 45 cm; at larger distances, a significant backscattered neutron component was seen. The measured backscattered component appears to be spatially uniform, and agrees with predictions of a neutron-transport code. As an additional application of the calibration results, it is shown that the neutron-induced signal in CR-39 used in charged-particle spectrometers on OMEGA can be used to determine DD and DT yields ranging from about 10 10 up to 10 14. With further improvements in the processing and analysis of CR-39, this upper limit can be increased by at least two orders of magnitude.

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The synthesis of several new families of misfit-layered compounds is demonstrated. These compounds are crystalline along the c-axis and in the ab-plane, but show very short coherence lengths between ab-planes. This disorder leads to ultra-low and tunable thermal conductivity. Annealing iso-

Turbostratically disordered tungsten diselenide (WSe 2 ) thin films with as few as two c-axisoriented (basal plane) structural units were synthesized from modulated elemental reactants. By varying the number of elemental W-Se bilayers deposited, the thickness could be controllably varied from two to eighty such structural units. The sample roughness decreases with increasing annealing time and temperature as the crystalline WSe 2 basal plane units self-assemble from the amorphous precursors. Low-angle X-ray diffraction data show that the thickness of the WSe 2 films is highly uniform after annealing, with estimated roughness of less than 0.2 nm, and highly oriented, with the c axis of the structural units oriented within 0.1°of the substrate normal as determined from rocking curves of the specular 00L-type diffraction peaks. Pole figures of hk0-type reflections indicate that c-axis-oriented basal plane structural units are randomly oriented within the a-b plane. The widths of diffraction peaks of type hk0, 00L, and hkl (h, k 6 ¼ 0; l 6 ¼ 0) indicate coherence lengths of about 6-7 nm in the a-b plane, the full thickness of the film along the c axis, and 1-2 nm in mixed-index directions. Scanning transmission electron microscopy imaging corroborated the X-ray scattering results, providing direct evidence of strong c-axis texture, rotational disorder between adjacent basal plane structural units, and an intraplanar grain size of several nanometers. The combination of intraplanar crystallinity and interplanar rotational disorder explains the significant anisotropy of the thermal conductivity, which is 20-30 times higher in the a-b plane than along the c axis. Electrical measurements within the a-b plane indicate that the films exhibit n-type semiconducting behavior.