An official website of the United States government
Carbon nanotubes (CNTs) are quasi-one dimensional nanostructures that display both high thermal conductivity for potential thermal management applications and intriguing low-dimensional phonon transport phenomena. In comparison to the advances made in the theoretical calculation of the lattice thermal conductivity of CNTs, thermal transport measurements of CNTs have been limited by either the poor temperature sensitivity of Raman thermometry technique or the presence of contact thermal resistance errors in sensitive two-probe resistance thermometry measurements. Here we report advances in a multi-probe measurement of the intrinsic thermal conductivity of individual multi-walled CNT samples that are transferred from the growth substrate onto the measurement device. The sample-thermometer thermal interface resistance is directly measured by this multi-probe method and used to model the temperature distribution along the contacted sample segment. The detailed temperature profile helps to eliminate the contact thermal resistance error in the obtained thermal conductivity of the suspended sample segment. A differential electro-thermal bridge measurement method is established to enhance the signal-to-noise ratio and reduce the measurement uncertainty by over 40%. The obtained thermal resistances of multiple suspended segments of the same MWCNT samples increase nearly linearly with increasing length, revealing diffusive phonon transport as a result of phonon-defect scattering in these MWCNT samples. The measured thermal conductivity increases with temperature and reaches up to 390 ± 20 W m-1 K-1 at room temperature for a 9-walled MWCNT. Theoretical analysis of the measurement results suggests submicron phonon mean free paths due to extrinsic phonon scattering by extended defects such as grain boundaries. The obtained thermal conductivity is decreased by a factor of 3 upon electron beam damage and surface contamination of the CNT sample.
more »
« less
Simultaneous Characterization of In-Plane and Cross-Plane Resistivities in Highly Anisotropic 2D Layered Heterostructures
Understanding and characterizing the intrinsic properties of charge carrier transport across the interfaces in van der Waals heterostructures is critical to their applications in modern electronics, thermoelectrics, and optoelectronics. However, there are very few published cross-plane resistivity measurements of thin samples because these inherently 2-probe measurements must be corrected for contact and lead resistances. Here, we present a method to extract contact resistances and metal lead resistances by fitting the width dependence of the contact end voltages of top and bottom electrodes of different linewidths to a model based on current crowding. These contributions are then subtracted from the total 2-probe cross-plane resistance to obtain the cross-plane resistance of the material itself without needing multiple devices and/or etching steps. This approach was used to measure cross-plane resistivities of a (PbSe)1(VSe2)1 heterostructure containing alternating layers of PbSe and VSe2 with random in-plane rotational disorder. Several samples measured exhibited a 4 order of magnitude difference between cross-plane and in-plane resistivities over the 6–300 K temperature range. We also reported the first observation of charge density wave transition in the cross-plane transport of (PbSe)1(VSe2)1 heterostructure. The device fabrication process is fully lift-off compatible, and the method developed enables the straightforward measurement of the resistivity anisotropy of most thin film materials with nm thicknesses.
more »
« less
- PAR ID:
- 10546807
- Publisher / Repository:
- ACS
- Date Published:
- Journal Name:
- ACS Nano
- Volume:
- 18
- Issue:
- 37
- ISSN:
- 1936-0851
- Page Range / eLocation ID:
- 25405 to 25413
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Thin (40–150 nm), highly doped n+ (1019–1020 cm−3) Ga2O3 layers deposited using pulsed laser deposition (PLD) were incorporated into Ti/Au ohmic contacts on (001) and (010) β-Ga2O3 substrates with carrier concentrations between 2.5 and 5.1 × 1018 cm−3. Specific contact resistivity values were calculated for contact structures both without and with a PLD layer having different thicknesses up to 150 nm. With the exception of a 40 nm PLD layer on the (001) substrate, the specific contact resistivity values decreased with increasing PLD layer thickness: up to 8× on (001) Ga2O3 and up to 16× on (010) Ga2O3 compared with samples without a PLD layer. The lowest average specific contact resistivities were achieved with 150 nm PLD layers: 3.48 × 10−5 Ω cm2 on (001) Ga2O3 and 4.79 × 10−5 Ω cm2 on (010) Ga2O3. Cross-sectional transmission electron microscopy images revealed differences in the microstructure and morphology of the PLD layers on the different substrate orientations. This study describes a low-temperature process that could be used to reduce the contact resistance in Ga2O3 devices.more » « less
-
Abstract An unusual self‐assembly pattern is observed for highly ordered 1500‐nm‐thick films of monodisperse 13‐nm‐sized colloidal PbSe quantum dots, originating from their faceted truncated cube‐like shape. Specifically, self‐assembled PbSe dots exhibited attachment to the substrate by <001> planes followed by an interconnection through the {001} facets in plan‐view and {110}/{111} facets in cross‐sectional‐view, thus forming a cubic superlattice. The thermoelectric properties of the PbSe superlattice thin films are investigated by means of frequency domain thermoreflectance, scanning thermal probe microscopy, and four‐probe measurements, and augmented by computational efforts. Thermal conductivity of the superlattice films is measured as low as 0.7 W m−1 K−1at room temperature due to the developed nanostructure. The low values of electrical conductivity are attributed to the presence of insulating oleate capping ligands at the dots’ surface and the small contact area between the PbSe dots within the superlattice. Experimental efforts aiming at the removal of the oleate ligands are conducted by annealing or molten‐salt treatment, and in the latter case, yielded a promising improvement by two orders of magnitude in thermoelectric performance. The result indicates that the straightforward molten‐salt treatment is an interesting approach to derive thermoelectric dot superlattice thin films over a centimeter‐sized area.more » « less
-
We present magnetic characterization, charge resistivity, and optical photoluminescence measurements on amorphous yttrium iron oxide thin films (a-Y–Fe–O), with supporting comparisons to amorphous germanium (a-Ge) films. We measured magnetic properties with both SQUID magnetometry and polarized neutron reflectometry. These results not only confirm that a-Y–Fe–O is a disordered magnetic material with strong predominantly antiferromagnetic exchange interactions and a high degree of frustration, but also that it is best understood electrically as a disordered semiconductor. As with amorphous germanium, a-Y–Fe–O obeys expectations for variable-range hopping through localized electron states over a wide range of temperature. We also clarify the consequences of charge transport through such a semiconducting medium for non-local voltage measurements intended to probe spin transport in nominally insulating magnetic materials. We further compare non-local resistance measurements made with “quasi-dc” automated current reversal to ac measurements made with a lock-in amplifier. These show that the “quasi-dc” measurement has an effective ac current excitation with frequency up to approximately 22 Hz, and that this effective ac excitation can cause artifacts in these measurements including incorrect sign of the non-local resistance. This comprehensive investigation of non-local resistance measurements in a-Y–Fe–O shows no evidence of spin transport on micrometer length scales, which is contrary to our original work, and in line with more recent investigations by other groups.more » « less
-
null (Ed.)Solution-processed graphene inks that use ethyl cellulose as a polymer stabilizer are blade-coated into large-area thin films. Following blade-coating, the graphene thin films are cured to pyrolyze the cellulosic polymer, leaving behind an sp 2 -rich amorphous carbon residue that serves as a binder in addition to facilitating charge transport between graphene flakes. Systematic charge transport measurements, including temperature-dependent Hall effect and non-contact microwave resonant cavity characterization, reveal that the resulting electrically percolating graphene thin films possess high mobility (≈160 cm 2 V −1 s −1 ), low energy gap, and thermally activated charge transport, which develop weak localization behavior at cryogenic temperatures.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acsnano.3c13232
