At the level of individual molecules, familiar concepts of heat transport no longer apply. When large amounts of heat are transported through a molecule, a crucial process in molecular electronic devices, energy is carried by discrete molecular vibrational excitations. We studied heat transport through self-assembled monolayers of long-chain hydrocarbon molecules anchored to a gold substrate by ultrafast heating of the gold with a femtosecond laser pulse. When the heat reached the methyl groups at the chain ends, a nonlinear coherent vibrational spectroscopy technique detected the resulting thermally induced disorder. The flow of heat into the chains was limited by the interface conductance. The leading edge of the heat burst traveled ballistically along the chains at a velocity of 1 kilometer per second. The molecular conductance per chain was 50 picowatts per kelvin.H eat transport is central to the operation of mechanical and electronic machinery, but at the level of individual molecules, the familiar concepts of heat diffusion by phonons in bulk materials no longer apply. Heat is transported through a molecule by discrete molecular vibrations. An emerging area in which vibrational energy transfer becomes crucial is the field of molecular electronics, where longchain molecules attached to tiny electrodes are used to transport and switch electrons. When an electron is transported through a molecule, a portion of the electron's kinetic energy can be lost, appearing as molecular vibrational energy (1). In studies such as this one, in which molecular energy levels are not individually resolved, it is conventional to call such processes "heat dissipation" or "nanoscale thermal transport" (2), even though an equilibrium Boltzmann distribution is not necessarily achieved. Nitzan and co-workers (3) have estimated that 10 to 50% of the electron energies could be converted to heat, so that a power of 10 11 eV/s may be dissipated on a molecular electronic bridge carrying 10 nA under a bias of 1 eV. Using classical and quantum mechanical methods, they and others (1) have calculated steadystate temperatures resulting from such dissipation. Steady-state calculations, however, do not entirely capture the essence of this phenomenon. The energy lost when electrons are transported through a molecular wire in a fraction of a picosecond appears as staccato bursts, up to 1 eV per burst. On a 10-carbon alkane molecule, for instance, 1eV is enough energy to produce a transient temperature jump DT ≈ 225 K. At the temperatures associated with these ultrafast energy bursts, Nitzan and coworkers (3) suggest that, instead of the usual phonon mechanisms prevalent in ordinary thermal conduction processes (1), much of the heat is carried by higher-energy molecular vibrations such as carbon-carbon bending and stretching and carbon-hydrogen bending, which are delocalized over a few carbon segments (3).To study molecular energy transport in the regime of short distances, short time intervals, and large temperature bursts, we have used an ultr...

The distribution of phonons that carry heat in crystals has typically been studied through measurements of the thermal conductivity Λ as a function of temperature or sample-size. We find that Λ of semiconductor alloys also depends on the frequency of the oscillating temperature field used in the measurement and hence demonstrate a novel and experimentally convenient probe of the phonon distribution. We report the frequency dependent Λ of In 0.49 Ga 0.51 P, In 0.53 Ga 0.47 As, and Si 0.4 Ge 0.6 as measured by time-domain thermoreflectance over a wide range of modulation frequencies, 0.1

We report the thermal conductance G of Au/Ti/graphene/SiO(2) interfaces (graphene layers 1 ≤ n ≤ 10) typical of graphene transistor contacts. We find G ≈ 25 MW m(-2) K(-1) at room temperature, four times smaller than the thermal conductance of a Au/Ti/SiO(2) interface, even when n = 1. We attribute this reduction to the thermal resistance of Au/Ti/graphene and graphene/SiO(2) interfaces acting in series. The temperature dependence of G from 50 ≤ T ≤ 500 K also indicates that heat is predominantly carried by phonons through these interfaces. Our findings suggest that metal contacts can limit not only electrical transport but also thermal dissipation from submicrometer graphene devices.

The heat transport mechanisms in superlattices are identified from the cross‐plane thermal conductivity Λ of (AlN)x–(GaN)y superlattices measured by time‐domain thermoreflectance. For (AlN)4.1 nm–(GaN)55 nm superlattices grown under different conditions, Λ varies by a factor of two; this is attributed to differences in the roughness of the AlN/GaN interfaces. Under the growth condition that gives the lowest Λ, Λ of (AlN)4 nm–(GaN)y superlattices decreases monotonically as y decreases, Λ = 6.35 W m−1 K−1 at y = 2.2 nm, 35 times smaller than Λ of bulk GaN. For long‐period superlattices (y > 40 nm), the mean thermal conductance G of AlN/GaN interfaces is independent of y, G ≈ 620 MW m−2 K−1. For y < 40 nm, the apparent value of G increases with decreasing y, reaching G ≈ 2 GW m−2 K−1 at y < 3 nm. MeV ion bombardment is used to help determine which phonons are responsible for heat transport in short period superlattices. The thermal conductivity of an (AlN)4.1 nm–(GaN)4.9 nm superlattice irradiated by 2.3 MeV Ar ions to a dose of 2 × 1014 ions cm−2 is reduced by <35%, suggesting that heat transport in these short‐period superlattices is dominated by long‐wavelength acoustic phonons. Calculations using a Debye‐Callaway model and the assumption of a boundary scattering rate that varies with phonon‐wavelength successfully capture the temperature, period, and ion‐dose dependence of Λ.

We demonstrate a reliable technique for counting atomic planes (n) of few-layer graphene (FLG) on SiO(2)/Si substrates by Raman spectroscopy. Our approach is based on measuring the ratio of the integrated intensity of the G graphene peak and the optical phonon peak of Si, I(G)/I(Si), and is particularly useful in the range n > 4 where few methods exist. We compare our results with atomic force microscopy (AFM) measurements and Fresnel equation calculations. Then, we apply our method to unambiguously identify n of FLG devices on SiO(2) and find that the mobility (μ ≈ 2000 cm(2) V(-1) s(-1)) is independent of layer thickness for n > 4. Our findings suggest that electrical transport in gated FLG devices is dominated by carriers near the FLG/SiO(2) interface and is thus limited by the environment, even for n > 4.

Thermal transport in layered, two-dimensional (2D) black phosphorus (BP) is of great interest, not only due to its importance in the designs of BP devices, [1] but also because it provides a unique platform to study the physics of heat transport in highly anisotropic materials. [2] BP belongs to the orthorhombic Cmca point group, [3] with its puckered honeycomb basal planes weakly bonded together by interlayer van der Waals'forces. Due to the nature of its crystal structure, second order tensors (e.g., the thermal conductivity tensor Λ) of BP have three independent components along the principal axes of zigzag (ZZ), armchair (AC) and through-plane (TP), see Figure 1a, and the thermal conductivity tensor is strongly anisotropic along these axes. [4] (In this paper, we use ΛZZ, ΛAC and ΛTP to denote the three independent components of the thermal conductivity tensor.) Here, we accurately measured and report the anisotropic thermal conductivity tensor (ΛZZ, ΛAC and ΛTP) of bulk BP in a temperature range of 80 ≤ T ≤ 300 K. Our temperature dependence measurements provide a crucial benchmark for future studies of anisotropic heat transport in BP and phosphorene.To date, there are only few experimental works on anisotropic thermal conductivity of BP, even at 300 K. Luo et al. [5] and Lee et al. [6] measured BP flakes with a thickness of 9 -30 nm and 60 -310 nm using the opto-thermal Raman method and the micro-bridge technique, respectively, and reported ΛZZ = 11 -45 W m -1 K -1 and ΛAC = 5 -22 W m -1 K -1 at room temperature. These values of ΛZZ and ΛAC are substantially lower than predictions by first-principles calculations [4, 7, 8] for bulk BP and phosphorene. While these low values of thermal conductivity were attributed to additional boundary scattering of phonons in the thin flakes, [5, 6] we note that scattering of phonons along the basal planes by the interfaces is rather weak [9] and thus this explanation might not be satisfactory. The low values could also originate from degradation of the BP flakes by oxidation, [10] as the BP flakes in both studies were exposed to the air for a substantial amount of time during sample preparation and measurements. With the degradation, the reported thermal conductivity is probably not intrinsic. Jang et al. [11] encapsulated their BP flakes of thickness of 138 -552 nm with a 3- Zhu et al.'s samples were not seriously oxidized, their pump-probe measurements in the through-plane direction might be lower than the intrinsic ΛTP because the mean-free-paths () of a substantial portion of heat-carrying phonons are much longer than the characteristic length scales of their measurements (<500 nm), i.e., the thickness of the samples or the thermal penetration depth d. [12][13][14] In fact, we obtained a ΛTP value ~25 % higher than Jang et al. 's and Zhu et al.'s measurements, [4, 11] when we used a much lower modulation frequency in our measurements to achieve a larger thermal penetration depth.With the relatively few published works on the thermal properties of BP, kn...

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.

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