Zhaoli Gao

Fabrication of carbon nanotube thermal interface material on aluminum alloy substrates with low pressure CVD.

High quality vertically aligned carbon nanotube (VACNT) arrays have been synthesized on bulk Al alloy (Al6063) substrates with an electron-beam (E-beam) evaporated Fe catalyst using low pressure chemical vapor deposition (LPCVD). The pretreatment process of the catalyst was shown to play a critical role. This was studied comprehensively and optimized to repeatedly grow high quality VACNT arrays within a wide range of thicknesses of catalyst layer (2-11 nm) and acetylene (C(2)H(2)) flow rates (100-300 sccm). The thermal performance of the resulting VACNT arrays was evaluated. The minimum interfacial thermal resistance of the Si/VACNT/Al interfaces achieved so far is only 4 mm(2) K W(-1), and the average value is 14.6 mm(2) K W(-1).

Scalable Production of Sensor Arrays Based on High-Mobility Hybrid Graphene Field Effect Transistors.

We have developed a scalable fabrication process for the production of DNA biosensors based on gold nanoparticle-decorated graphene field effect transistors (AuNP-Gr-FETs), where monodisperse AuNPs are created through physical vapor deposition followed by thermal annealing. The FETs are created in a four-probe configuration, using an optimized bilayer photolithography process that yields chemically clean devices, as confirmed by XPS and AFM, with high carrier mobility (3590 ± 710 cm2/V·s) and low unintended doping (Dirac voltages of 9.4 ± 2.7 V). The AuNP-Gr-FETs were readily functionalized with thiolated probe DNA to yield DNA biosensors with a detection limit of 1 nM and high specificity against noncomplementary DNA. Our work provides a pathway toward the scalable fabrication of high-performance AuNP-Gr-FET devices for label-free nucleic acid testing in a realistic clinical setting.

Thermal performance of LED packages for solid state lighting with novel cooling solutions

With the increasing application of high power LEDs in general lighting, more effective cooling solutions should be considered to maintain a better performance and reliability with lower LED junction temperature. In this paper, it is discussed firstly in detail how to effectively take advantage of high thermal performance materials, such as CNTs, to improve the heat conduction in LED packages. Secondly, the air flow velocity field generated by piezoelectric fans is simulated using 3D fluid structure interaction method (FSI) and verified with experimental data. A cooler with a piezoelectric fan inside is designed as a preliminary study on how to apply piezoelectric fans in LED active cooling.

Graphene heat spreader for thermal management of hot spots

Monolayer graphene was fabricated using thermal chemical vapor deposition (TCVD) as heat spreaders in electronic packaging. Platinum (Pt) thermal evaluation chips were utilized to evaluate the thermal performance of the graphene heat spreaders. Temperature of hot spot driven at a heat flux of up to 430W·cm-2 was decreased by about 13 °C with the attaching of the graphene heat spreader. We demonstrate the potentials of using CMOS compatible TCVD process to make graphene as heat spreader for power dissipation needs.

Recoil Effect and Photoemission Splitting of Trions in Monolayer MoS2

The 2D geometry nature and low dielectric constant in transition-metal dichalcogenides lead to easily formed strongly bound excitons and trions. Here, we studied the photoluminescence of van der Waals heterostructures of monolayer MoS2 and graphene at room temperature and observed two photoluminescence peaks that are associated with trion emission. Further study of different heterostructure configurations confirms that these two peaks are intrinsic to MoS2 and originate from a bound state and Fermi level, respectively, of which both accept recoiled electrons from trion recombination. We demonstrate that the recoil effect allows us to electrically control the photon energy of trion emission by adjusting the gate voltage. In addition, significant thermal smearing at room temperature results in capture of recoil electrons by bound states, creating photoemission peak at low doping level whose photon energy is less sensitive to gate voltage tuning. This discovery reveals an unexpected role of bound states for photoemission, where binding of recoil electrons becomes important.

Fabrication of carbon nanotube thermal interface material on aluminium alloy substrates

High quality vertically aligned carbon nanotube (VACNT) arrays have been synthesized on bulk Al alloy (Al6063) substrates with an Fe catalyst using low pressure chemical vapor deposition (LPCVD). The pretreatment process of the catalyst was shown to play a critical role, and it was studied comprehensively and optimized to grow high quality VACNT arrays within a wide range thicknesses of catalyst layer (2nm – 11 nm) and acetylene (C2H2) flow rates (100sccm – 300sccm). The thermal performance of the resulting VACNT arrays is evaluated. The minimum thermal interfacial resistance of the Al-CNT-Si interfaces achieved so far is only 4 mm2·K/W, and the average value is 14.6 mm2·K/W.

Highly conductive die attach adhesive from percolation control and its applications in light-emitting device thermal management

Herein, we reported on the study of percolation dynamics in thermoset-based die attach (DA) materials and its effect on percolation conductivity. Two types of percolation mechanism in thermoset based DA were discovered, i.e., the curing reaction-induced percolation and the physical aging-induced percolation. The former features in a fast percolation network growth rate, which is one order of magnitude higher than the latter. It is demonstrated that the percolation kinetics largely affects the apparent percolation conductivity under the traditional packaging conditions; and reaction-induced percolation allows ultrahigh efficiency in reaching the volume fraction-limiting percolation conductance, resulting in enhanced thermal performance of DA.

Novel cooling solutions for LED solid state lighting

With more and more high power high brightness LEDs adopted in general lighting applications, the state-of-the-art packages cannot meet the requirement of thermal management. Over-heating of an LED will cause premature failure because the efficiency, the spectrum, the reliability and the life of solid state lighting devices strongly depend on successful thermal management. Therefore, the requirement for improving the performance and the reliability poses a significant challenge for novel cooling solutions including applications of heat sinks with active cooling methods and high thermal performance materials. Conventional mechanical fans are not preferred in LED devices due to its high energy consumption, short life and high noise. In this study, cooling fixtures with piezoelectric units are designed to be integrated with high power LED modules. The cooling effect of high performance TIM and piezoelectric units are investigated using numerical modeling method.

Performance of high-brightness LEDs with VACNT-TIM on aluminum heat spreaders

An application using vertically aligned carbon nanotube (VACNT) as thermal interface material (TIM) in high-brightness LEDs is demonstrated. VACNT arrays have been synthesized on the top and the bottom surfaces of the aluminum heat slugs and integrated into the LED modules using the designed packaging process. The thermal resistance of the LEDs is characterized using the thermal transient test system (MicRed, T3Ster). Compared with silver epoxy, VACNT-TIM shows a comparable thermal performance. It is found that a proper packaging process plays an important role in the final thermal performance of the LEDs with VACNT-TIM.