August 27, 2019

Our very first PhD student Anh Tuan Nguyen joined the group!! 

July 3, 2019

Abstract: Considerable advances in manipulating heat flow in solids have been made through the innovation of artificial thermal structures such as thermal diodes, camouflages, and cloaks. Such thermal devices can be readily constructed only at the macroscale by mechanically assembling different materials with distinct values of thermal conductivity. Here, we extend these concepts to the microscale by demonstrating a monolithic material structure on which nearly arbitrary microscale thermal metamaterial patterns can be written and programmed. It is based on a single, suspended silicon membrane whose thermal conductivity is locally, continuously, and reversibly engineered over a wide range (between 2 and 65 W/m·K) and with fine spatial resolution (10–100 nm) by focused ion irradiation. Our thermal cloak demonstration shows how ion-write microthermotics can be used as a lithography-free platform to create thermal metamaterials that control heat flow at the microscale.


March 18, 2019

Abstract: The field of thermoelectric research has undergone a renaissance and boom in the past two and a half decades, largely fueled by the prospect of engineering electronic and phononic properties in nanostructures, among which semiconductor nanowires (NWs) have served both as an important platform to investigate fundamental thermoelectric transport phenomena and as a promising route for high thermoelectric performance for diverse applications. In this Review, we provide a comprehensive look at various aspects of thermoelectrics of NWs. We start with a brief introduction of basic thermoelectric phenomena, followed by synthetic methods for thermoelectric NWs and a summary of their thermoelectric figures of merit (ZT). We then focus our discussion on charge and heat transport, which dictate thermoelectric power factor and thermal conductivity, respectively. For charge transport, we cover the basic principles governing the power factor and then review several strategies using NWs to e...

August 20, 2018

Abstract: Functional semiconductor heterojunctions are fundamental units for building up advanced optoelectronics and circuits. Halide perovskites, representing a new class of semiconductors with soft and reconfigurable ionic bonding, hold promise for a variety of applications because of their many unusual, tunable physical properties. This paper reports the formation of the current-rectifying p-n heterojunction in single-crystalline CsSnI3 nanowires via localized phase transition between the n-type yellow and p-type black phases. We attribute the distinction of majority carrier types in these two phases to the different formation energies of the cation and anion vacancies. The present approach to heterojunction formation could inspire deeper understanding of phase-transition dynamics and enable precise control over the design of functional heterostructures using halide perovskite building blocks.


May 20, 2018

Abstract: Alloying different semiconductors is a powerful approach to tuning the optical and electronic properties of semiconductor materials. In halide perovskites (ABX3), alloys with different anions have been widely studied, and great band gap tunability in the visible range has been achieved. However, perovskite alloys with different cations at the “B” site are less understood due to the synthetic challenges. Herein, we first have developed the synthesis of single-crystalline CsPbxSn1–xI3 nanowires (NWs). The electronic band gaps of CsPbxSn1–xI3 NWs can be tuned from 1.3 to 1.78 eV by varying the Pb/Sn ratio, which leads to the tunable photoluminescence (PL) in the near-infrared range. More importantly, we found that the electrical conductivity increases as more Sn2+ is alloyed with Pb2+, possibly due to the increase of charge carrier concentration when more Sn2+ is introduced. The wide tunability of the optical and electronic properties makes CsPbxSn1–xI3 alloy NWs promising candi...

January 3, 2018

Officially started a faculty duty from January 2018!

August 21, 2017

Our paper about thermal and thermoelectric properties of all-inorganic halide perovskite nanowires has been published on PNAS. We Discovered ultralow thermal conductivity (~0.4 W/m∙K), comparable to their amorphous limit in single-crystalline all-inorganic halide perovskite nanowires and attributed it to a cluster rattling mechanism. Black-phase CsSnI3 shows a rare combination of ultralow thermal conductivity and high electrical conductivity, resulting in ZT of 0.11 around room temperature. This study will open up opportunities for energy conversion applications and call attention to the thermal management strategies for applications with all-inorganic halide perovskites. This work has been highlighted by LBNL news and others. 

August 20, 2017

A new group website is created and launched. 

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© 2019 by Woochul Lee