Zuanyi Li

Ballistic to diffusive crossover of heat flow in graphene ribbons

Heat flow in nanomaterials is an important area of study, with both fundamental and technological implications. However, little is known about heat flow in two-dimensional devices or interconnects with dimensions comparable to the phonon mean free path. Here we find that short, quarter-micron graphene samples reach ~35% of the ballistic thermal conductance limit up to room temperature, enabled by the relatively large phonon mean free path (~100 nm) in substrate-supported graphene. In contrast, patterning similar samples into nanoribbons leads to a diffusive heat-flow regime that is controlled by ribbon width and edge disorder. In the edge-controlled regime, the graphene nanoribbon thermal conductivity scales with width approximately as ~W(1.8)(0.3), being about 100 W m(-1) K(-1) in 65-nm-wide graphene nanoribbons, at room temperature. These results show how manipulation of two-dimensional device dimensions and edges can be used to achieve full control of their heat-carrying properties, approaching fundamentally limited upper or lower bounds.

Role of Joule Heating on Current Saturation and Transient Behavior of Graphene Transistors

We use simulations to examine current saturation in sub-micron graphene transistors on SiO2/Si. We find that self-heating is partly responsible for current saturation (lower output conductance) but degrades current densities above 1 bmA/μm by up to 15%. Heating effects are reduced if the supporting insulator is thinned or, in shorter channel devices, by partial heat sinking at the contacts. The transient behavior of such devices has thermal time constants of ~ 30-300 ns, which is dominated by the thickness of the supporting insulator and that of the device capping layers (a behavior also expected in ultrathin-body SOI transistors). The results shed important physical insight into the high-field and transient behavior of graphene transistors.

Carbon Nanotube Circuit Integration up to Sub-20 nm Channel Lengths

Carbon nanotube (CNT) field-effect transistors (CNFETs) are a promising emerging technology projected to achieve over an order of magnitude improvement in energy-delay product, a metric of performance and energy efficiency, compared to silicon-based circuits. However, due to substantial imperfections inherent with CNTs, the promise of CNFETs has yet to be fully realized. Techniques to overcome these imperfections have yielded promising results, but thus far only at large technology nodes (1 μm device size). Here we demonstrate the first very large scale integration (VLSI)-compatible approach to realizing CNFET digital circuits at highly scaled technology nodes, with devices ranging from 90 nm to sub-20 nm channel lengths. We demonstrate inverters functioning at 1 MHz and a fully integrated CNFET infrared light sensor and interface circuit at 32 nm channel length. This demonstrates the feasibility of realizing more complex CNFET circuits at highly scaled technology nodes.

Electrical and Thermoelectric Transport by Variable Range Hopping in Thin Black Phosphorus Devices

The moderate band gap of black phosphorus (BP) in the range of 0.3-2 eV, along a high mobility of a few hundred cm(2) V(-1) s(-1) provides a bridge between the gapless graphene and relatively low-mobility transition metal dichalcogenides. Here, we study the mechanism of electrical and thermoelectric transport in 10-30 nm thick BP devices by measurements of electrical conductance and thermopower (S) with various temperatures (T) and gate-electric fields. The T dependences of S and the sheet conductance (σ□) of the BP devices show behaviors of T(1/3) and exp[-(1/T)(1/3)], respectively, where S reaches ∼0.4 mV/K near room T. This result indicates that two-dimensional (2D) Motts variable range hopping (VRH) is a dominant mechanism in the thermoelectric and electrical transport in our examined thin BP devices. We consider the origin of the 2D Motts VRH transport in our BPs as trapped charges at the surface of the underlying SiO2 based on the analysis with observed multiple quantum dots.

Thermal conductivity of chirality-sorted carbon nanotube networks

The thermal properties of single-walled carbon nanotubes (SWNTs) are of significant interest, yet their dependence on SWNT chirality has been, until now, not explored experimentally. Here, we used electrical heating and infrared thermal imaging to simultaneously study thermal and electrical transport in chirality-sorted SWNT networks. We examined solution processed 90% semiconducting, 90% metallic, purified unsorted (66% semiconducting), and as-grown HiPco SWNT films. The thermal conductivities of these films range from 80 to 370 W m−1 K−1 but are not controlled by chirality, instead being dependent on the morphology (i.e., mass and junction density, quasi-alignment) of the networks. The upper range of the thermal conductivities measured is comparable to that of the best metals (Cu and Ag), but with over an order of magnitude lower mass density. This study reveals important factors controlling the thermal properties of light-weight chirality-sorted SWNT films, for potential thermal and thermoelectric appl...

Reliability, failure, and fundamental limits of graphene and carbon nanotube interconnects

We review recent results concerning reliability and failure (due to heating) of interconnects based on metallic single-walled carbon nanotubes (SWNTs), graphene, and graphene nanoribbons (GNRs). We examine both intrinsic power dissipation within the interconnect as well as extrinsically to adjacent materials. Fundamental reliability limits are different in the diffusive and quasi-ballistic transport regimes. Thermal engineering in the diffusive regime has recently enabled us to reach current densities up to ~4 GA/cm2 for SWNTs and ~2 GA/cm2 for GNRs. However, short carbon-based interconnects (e.g. L <; 500 nm) enter a quasi-ballistic regime where heat sinking is dominated by the contacts, and narrow GNRs are in an edge-limited regime (W <; 200 nm), where thermal conductivity is dependent on device dimensions.

High-field and thermal transport in 2D atomic layer devices

This paper reviews our recent results of high-field electrical and thermal properties of atomically thin two-dimensional materials. We show how self-heating affects velocity saturation in suspended and supported graphene. We also demonstrate that multi-valley transport must be taken into account to describe high-field transport in MoS2. At the same time we characterized thermal properties of suspended and nanoscale graphene samples over a wide range of temperatures. We uncovered the effects of edge scattering and grain boundaries on thermal transport in graphene, and showed how the thermal conductivity varies between diffusive and ballistic heat flow limits.

Thermal transport across graphene step junctions

Step junctions are often present in layered materials, i.e. where single-layer regions meet multi-layer regions, yet their effect on thermal transport is not understood to date. Here, we measure heat flow across graphene junctions (GJs) from monolayer to bilayer graphene, as well as bilayer to four-layer graphene for the first time, in both heat flow directions. The thermal conductance of the monolayer-bilayer GJ device ranges from ~0.5 to 9.1x10^8 Wm-2K-1 between 50 K to 300 K. Atomistic simulations of such GJ device reveal that graphene layers are relatively decoupled, and the low thermal conductance of the device is determined by the resistance between the two dis-tinct graphene layers. In these conditions the junction plays a negligible effect. To prove that the decoupling between layers controls thermal transport in the junction, the heat flow in both directions was measured, showing no evidence of thermal asymmetry or rectification (within experimental error bars). For large-area graphene applications, this signifies that small bilayer (or multilayer) islands have little or no contribution to overall thermal transport.

Device and energy properties of two-dimensional (2D) atomically thin materials

This talk will give an overview of our recent work on two-dimensional (2D) materials and devices. Particular focus will be placed on high-field transport, device self-heating, and fundamental aspects of thermal (phonon) transport in 2D materials including graphene and MoS2.

Energy efficiency and conversion in 1D and 2D electronics

We review our recent studies at the intersection of energy, nanomaterials and nanoelectronics. Through careful high-field studies of two-dimensional (2D) devices based on graphene and MoS2, we have uncovered details regarding their physical properties and band structure. We have investigated thermoelectric effects in graphene transistors and phase-change memory (PCM) elements for low-power electronics. We find that low-power transistors and memory could be enhanced by built-in thermoelectric effects which are particularly pronounced at nanometer length scales. We have also examined heat flow in composites based on one-dimensional (1D) carbon nanotubes, and uncovered both the lower (diffusive) and upper (ballistic) limits of heat flow in 1D and 2D nanomaterials. Our results suggest fundamental limits and new applications that could be achieved through the co-design of geometry, interfaces, and selection of 1D and 2D nanomaterials.