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and A. M. Basilio, Hsu YK, Wei PC, Ganguly A, Shih HC, Chen YT, Chen LC, Chen* KH.  2010.  Electrochemical Characterization of InN Thin Film for Biosensing Applications. J. New Mat. Electrochem. Systems. 13:337-343.
Amloy, S, Chen YT, Karlsson KF, Chen KH, Hsu HC, Hsiao CL, C.Chen L, Holtz* PO.  2011.  Polarization resolved fine structure splitting of zero-dimensional InGaN excitons. Phys. Rev. B. 83:201307.
Aravind, K, Su YW, Chun DS, Kuo W, Wu CS, Chang-Liao KS, Chen KH, Chen LC, Chen CD.  2012.  Magnetic-field and temperature dependence of the energy gap in InN nanobelt. AIP Advances. 2:012155.
Aravind, K, Su YW, Ho IL, Wu CS, Chang-Liao KS, Su WF, Chen KH, Chen LC, Chen CD.  2009.  Coulomb blockade behavior in an indium nitride nanowire with disordered surface states. Appl. Phys. Lett.. 95:092110-(1-3).
Basilio, AM, Hsu YK, Tu WH, Hsu GM, Chen LC, Chen* KH.  2010.  Improving the photoelectrochemical and H2gas generation property of c-plane GaN thin film through crystallographic etching of the film. J. Mater. Chem.. 20:8118-8125.
Bayikadi, KS, Sankar R, Wu CT, Xia C, Chen Y, Chen L-C, Chen K-H, Chou F-C.  2019.  Enhanced thermoelectric performance of GeTe through in situ microdomain and Ge-vacancy control, 2019. Journal of Materials Chemistry A. 7(25):15181-15189.: The Royal Society of Chemistry AbstractWebsite

A highly reproducible sample preparation method for pure GeTe in a rhombohedral structure without converting to the cubic structure up to ∼500 °C is reported to show control of the Ge-vacancy level and the corresponding herringbone-structured microdomains. The thermoelectric figure-of-merit (ZT) for GeTe powder could be raised from ∼0.8 to 1.37 at high temperature (HT) near ∼500 °C by tuning the Ge-vacancy level through the applied reversible in situ route, which made it highly controllable and reproducible. The enhanced ZT of GeTe was found to be strongly correlated with both its significantly increased Seebeck coefficient (∼161 μV K−1 at 500 °C) and reduced thermal conductivity (∼2.62 W m−1 K−1 at 500 °C) for a sample with nearly vacancy-free thicker herringbone-structured microdomains in the suppressed rhombohedral-to-cubic structure phase transformation. The microdomain and crystal structures were identified with HR-TEM (high-resolution transmission electron microscopy) and powder X-ray diffraction (XRD), while electron probe micro-analysis (EPMA) was used to confirm the stoichiometry changes of Ge : Te. Theoretical calculations for GeTe with various Ge-vacancy levels suggested that the Fermi level shifts toward the valence band as a function of increasing the Ge-vacancy level, which is consistent with the increased hole-type carrier concentration (n) and effective mass (m*) deduced from the Hall measurements. The uniquely prepared sample of a near-vacancy-free GeTe in a rhombohedral structure at high temperature favoured an enhanced Seebeck coefficient in view of the converging L- and Σ-bands of the heavy effective mass at the Fermi level, while the high density domain boundaries for the domain of low carrier density were shown to reduce the total thermal conductivity effectively.

Bayikadi, KS, Imam S, Ubaid M, Aziz A, Chen K-H, Sankar R.  2022.  Effect of aliovalent substituted highly disordered GeTe compound's thermoelectric performance, 2022. 922:166221. AbstractWebsite

As a lead-free high-performance thermoelectric material, germanium telluride (GeTe) has recently been extensively studied for mid-temperature (500–800 K) applications. The carrier concentration and the thermal conductivity are reduced for vacancy-controlled GeTe compounds compared with pristine GeTe. We explored and optimized the Ge0.9−xSb0.1PxTe (x = 0.01–0.05) material's highest thermoelectric performance at elevated temperatures. Intrinsic Ge vacancy control and manipulation of Ge (+2) with Sb/P (+3) increased the charge contribution to power factor improvement to ∼42 µWcm−1 K−2 while minimizing the lattice thermal contribution to ∼0.4 W/mK. This resulted in an increase in thermoelectric performance of ∼2.4 @ 773 K for the Ge0.88Sb0.1P0.02Te sample. The inclusion of atomically disordered Sb/P ions considerably increases the scattering effects caused by the point defect, whereas stretched grain boundaries reveal the decreased lattice thermal contribution. The current work demonstrates the effectiveness of phosphorus as a co-dopant in increasing the average thermoelectric performance (ZTavg) value over the GeTe operating temperature range.

Bayikadi, KS, Wu CT, Chen L-C, Chen K-H, Chou F-C, Sankar R.  2020.  Synergistic optimization of thermoelectric performance of Sb doped GeTe with a strained domain and domain boundaries, 2020. Journal of Materials Chemistry A. 8(10):5332-5341.: The Royal Society of Chemistry AbstractWebsite

In addition to the Ge-vacancy control of GeTe, the antimony (Sb) substitution of GeTe for the improvement of thermoelectric performance is explored for Ge1−xSbxTe with x = 0.08–0.12. The concomitant carrier concentration (n) and the aliovalent Sb ion substitution led to an optimal doping level of x = 0.10 to show ZT ∼ 2.35 near ∼800 K, which is significantly higher than those single- and multi-element substitution studies of the GeTe system reported in the literature. In addition, Ge0.9Sb0.1Te demonstrates an impressively high power factor of ∼36 μW cm−1 K−2 and a low thermal conductivity of ∼1.1 W m−1 K−1 at 800 K. The enhanced ZT level for Ge0.9Sb0.1Te is explained through a systematic investigation of micro-structural change and strain analysis from room temperature to 800 K. A significant reduction of lattice thermal conductivity (κlat) is identified and explained by the Sb substitution-introduced strained and widened domain boundaries for the herringbone domain structure of Ge0.9Sb0.1Te. The Sb substitution created multiple forms of strain near the defect centre, the herringbone domain structure, and widened tensile/compressive domain boundaries to support phonon scattering that covers a wide frequency range of the phonon spectrum to reduce lattice thermal conductivity effectively.

Berzina, B, Trinkler L, Jakimovica D, Korsaks V, Grabis J, Steins I, Palcevskis, Bellucci S, Chen LC, Chattopadhyay S t, Chen KH.  2009.  Spectral characterization of bulk and nanostructuredaluminum nitride. J. Nanophotonics. 3:031950.
Bhusari, DM, Yang JR, Wang TY, Chen KH, Lin ST, Chen LC.  1998.  Effect of Substrate Pretreatment and Methane Fraction on the Optical Transparency of Nano-crystalline Diamond Thin Films. J. Mater. Res.. 13:1769.
Bhusari, DM, Chen CK, Chen KH, Chuang TJ, Chen LC, Lin MC.  1997.  Composition of SiCN Crystals Consisting of a Predominantly Carbon-nitride Network. J. Mater. Res.. 12:322.
Bhusari, DM, Yang JR, Wang TY, Chen KH, Chen LC.  1998.  Novel Two Stage Method for Growth of Highly Transparent Nano-crystalline Diamond Films. Mater. Lett.. 36:279.
Bhusari, DM, Teng CW, Chen KH, Chen LC.  1997.  Traveling wave method for measurement of thermal conductivity of thin films. Rev. Sci. Instrum.. 68(11):4180-4183.
Bhusari, DM, Chen KH, Yang TR, Lin ST, Wang TY, Chen LC.  1998.  Highly transparent nano-crystalline diamond films grown by microwave CVD. Solid State Comm.. 107:301-305.
Billo, T, Fu F-Y, Raghunath P, Shown I, Chen W-F, Lien H-T, Shen T-H, Lee J-F, Chan T-S, Huang K-Y, Wu C-I, Lin MC, Hwang J-S, Lee C-H, Chen L-C, Chen K-H.  2018.  Ni-Nanocluster Modified Black TiO2 with Dual Active Sites for Selective Photocatalytic CO2 Reduction. Small. 14:1702928–n/a., Number 2 AbstractWebsite

One of the key challenges in artificial photosynthesis is to design a photocatalyst that can bind and activate the CO2 molecule with the smallest possible activation energy and produce selective hydrocarbon products. In this contribution, a combined experimental and computational study on Ni-nanocluster loaded black TiO2 (Ni/TiO2[Vo]) with built-in dual active sites for selective photocatalytic CO2 conversion is reported. The findings reveal that the synergistic effects of deliberately induced Ni nanoclusters and oxygen vacancies provide (1) energetically stable CO2 binding sites with the lowest activation energy (0.08 eV), (2) highly reactive sites, (3) a fast electron transfer pathway, and (4) enhanced light harvesting by lowering the bandgap. The Ni/TiO2[Vo] photocatalyst has demonstrated highly selective and enhanced photocatalytic activity of more than 18 times higher solar fuel production than the commercial TiO2 (P-25). An insight into the mechanisms of interfacial charge transfer and product formation is explored.

Billo, T, Shown I, kumar Anbalagan A, Effendi TA, Sabbah A, Fu F-Y, Chu C-M, Woon W-Y, Chen R-S, Lee C-H, Chen K-H, Chen L-C.  2020.  A mechanistic study of molecular CO2 interaction and adsorption on carbon implanted SnS2 thin film for photocatalytic CO2 reduction activity, 2020. 72:104717. AbstractWebsite

Gas-phase photocatalytic reactions to convert carbon dioxide and water into oxygen and hydrocarbons are the foundation of life on earth. However, the efficiency of photosynthesis is relatively low (~1%), which leaves much room for artificial photosynthesis to reach the benchmark of the solar cells (>15%). In this work, carbon implanted SnS2 thin films (C–SnS2) were prepared to study photocatalytic activity and adsorbate-catalyst surface interactions during CO2 photoreduction. The electron density distribution in C–SnS2 and its contribution toward the photogenerated charge transfer process has been analyzed by the angle-dependent X-ray absorption near-edge structure (XANES) study. The C–SnS2 surface affinity toward the CO2 molecule was monitored by in-situ dark current and Raman spectroscopy measurements. By optimizing the dose during ion implantation, SnS2 thin film with 1 wt% carbon incorporation shows 108 times enhancement in the CO2 conversion efficiency and more than 89% product selectivity toward CH4 formation compared with the as-grown SnS2 without carbon incorporation. The improved photocatalytic activity can be ascribed to enhanced light harvesting, pronounced charge-transfer between SnS2 and carbon with improved carrier separation and the availability of highly active carbon sites that serve as favorable CO2 adsorption sites.

Bloembergen, N, Chen KH, Lu CZ, Mazur E.  1990.  Multiplex Pure Rotational Coherent Anti-Stokes Raman Spectroscopy in a Molecular Beam. J. Raman Spectroscopy. 21:819.
C. Y. Chang, Chi GC, Wang WM, Chen LC, Chen KH, Ren F, Pearton* SJ.  2005.  Transport properties of InN nanowires. Appl. Phys. Lett.. 87:093112-(1-3).
and C.-C. Chen*, Yeh C-C, Liang C-H, Lee C-C, Chen C-H, Yu M-Y, Liu H-L, Chen LC, Lin YS, Ma KJ, Chen KH.  2001.  Preparation and characterization of carbon nanotubes encapsulated GaN nanowires. J. Phys. Chem. of Solids. 62:1577-1586.
and C.H. Lin, KH, Chattopadhyay S, Hsu CW, Wu MH, Chen WC, Wu CT, Tsen SC, Lee JH, Chen CH, Chen CW, Chen LC, Chen* KH.  2009.  Enhanced charge separation by sieve-layer mediation in high efficiency inorganic-organic solar cell. Adv. Mater.. 21:259-263.
Chang, YK, Hsieh HH, Pong WF, Tsai MH, Lee KH, Dann TE, Chien FZ, Tseng PK, Tsang KL, Su WK, Chen LC, Wei SL, Chen KH, Bhusari DM, Chen YF.  1998.  Electronic and Atomic Structures of SiCN Thin Film by X-ray Absorption Spectroscopy and Theoretical Calculations. Phys. Rev.. B58:9018.
Chang, H-C, You H-J, Sankar R, Yang Y-J, Chen L-C, Chen K-H.  2019.  Enhanced thermoelectric performance of BiCuTeO by excess Bi additions, 2019. 45(7, Part A):9254-9259. AbstractWebsite

Thermoelectric (TE) devices used to convert waste heat directly into electricity are highly desirable for alleviating the prevailing energy crisis and global climate-change issues. Among the various TE materials available, metal oxides exhibit high thermal and chemical stabilities in air, and are hence, preferred for use in many TE applications. However, most of them possess TE figures of merit (ZT) that are below the applicable value of 2, in the mid-temperature region (from 250 to 600 °C). In a previous work, the removal of a small amount of Bi from BiCuSeO was found to improve the ZT of BiCuSeO. In this work, we pursue another track and study the TE performance of BiCuTeO after the addition of up to 6% excess Bi. Bi1+xCuTeO (x = 0.00–0.06) samples were prepared by solid-state reactions, followed by hot-pressing to form pellets. By adding a stoichiometric excess of Bi into BiCuTeO, 16% enhancement in power factor was achieved at 450 °C. This enhancement can be attributed to the increase in the Seebeck coefficient because of the appearance of secondary phases. Detailed characterizations and discussions of the effect of the nominal excess Bi in BiCuTeO are presented in this paper. The findings of this study can be applied in the investigation of novel high-performance TE materials.

Chang, YS, Chien CT, Chen* CW, Chu TY, Chiang HH, Ku CH, Wu JJ, Lin CS, Chen LC, Chen KH.  2007.  Structural and optical properties of single crystal Zn1-xMgxO nanorods-experimental and theoretical studies. J. Appl. Phys.. 101:033502.
Chang, HJ, Chen CH, Chen* YF, Lin TY, Chen LC, Chen KH, Lan ZH.  2005.  Direct evidence of nanocluster-induced luminescence in InGaN epifilms. Appl. Phys. Lett.. 86:021911-(1-3).
Chang, CY, Pearton* SJ, Huang PJ, G.C. Chi H, Wang T, Chen JJ, Ren F, Chen KH, Chen LC.  2007.  Control of nucleation site density of GaN nanowires. Appl. Surf. Sci.. 253:3196-3200.