Yudi Darma

Visible Surface Plasmon Modes in Single Bi₂Te₃ Nanoplate.

Searching for new plasmonic building blocks which offer tunability and design flexibility beyond noble metals is crucial for advancing the field of plasmonics. Herein, we report that solution-synthesized hexagonal Bi2Te3 nanoplates, in the absence of grating configurations, can exhibit multiple plasmon modes covering the entire visible range, as observed by transmission electron microscopy (TEM)-based electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) spectroscopy. Moreover, different plasmon modes are observed in the center and edge of the single Bi2Te3 nanoplate and a breathing mode is discovered for the first time in a non-noble metal. Theoretical calculations show that the plasmons observed in the visible range are mainly due to strong spin-orbit coupling induced metallic surface states of Bi2Te3. The versatility of shape- and size-engineered Bi2Te3 nanocrystals suggests exciting possibilities in plasmonics-enabled technology.

Interplay of Cu and oxygen vacancy in optical transitions and screening of excitons in ZnO:Cu films

We study room temperature optics and electronic structures of ZnO:Cu films as a function of Cu concentration using a combination of spectroscopic ellipsometry, photoluminescence, and ultraviolet-visible absorption spectroscopy. Mid-gap optical states, interband transitions, and excitons are observed and distinguishable. We argue that the mid-gap states are originated from interactions of Cu and oxygen vacancy (Vo). They are located below conduction band (Zn4s) and above valence band (O2p) promoting strong green emission and narrowing optical band gap. Excitonic states are screened and its intensities decrease upon Cu doping. Our results show the importance of Cu and Vo driving the electronic structures and optical transitions in ZnO:Cu films.

Modeling and simulation of single electron transistor with master equation approach

In this paper, we discuss modeling and simulation of single dot Single Electron Transistor (SET) using master equation approximation. For SET modeling and simulation, master equation method treats the electron tunneling and its transition probabilistically. The probability of electron tunneling is used to determine the current density in accordance with selected input parameters. The calculation results show fairly accurate electrical characteristics of SET as compared with experimental data. Staircase pattern from I-V are clearly obtained as the main role of coulomb blockade effect in SET system. We also extend our calculation by introduce some additional parameters such as; the effect of working temperature, gate voltage dependent, and the influence of resistance to the device characteristic. We found that increasing operational temperature will promote higher current density, both in forward and reverse bias region. In the case of using single dot with 30 nm × 80 nm × 125 nm dimension, coulomb blockade effect could be reduced by applying gate voltage higher than 3V and setting drain resistance higher than sources. Our studies show an alternative approach in modeling and simulation of electronic devices and could be potential for development of novel nanoelectronic devices.

The coupling of single-photon exciton–biexciton quantum dot and cavity

We investigate the influence of multiple excitons on the photon emission properties of a quantum dot (QD)-cavity system via the master equation for the density matrix. We show that in the intermediate to strong coupling regimes, the multiple excitons lead to the suppressed QD emissions as well as the absence of anti-crossing near zero detuning, arising from the interaction between the multiple excitons and cavity. Furthermore, we analyze the role of the cavity-biexciton detuning in the photon emission properties of cavity and exciton through the second-order correlation function. The small cavity-biexciton detuning yields the significant Purcell effect and the high probability of single photon emissions. The proposed model offers the fundamental approach in developing efficient single-photon emitting devices.

Defect-induced excitonic recombination in Ti x Zn1− x O thin films grown by DC-unbalanced magnetron sputtering

We study the effects of Ti doping on the near-band-edge emission (NBE) and defect-related deep-level emission (DLE) of ZnO thin films grown by DC unbalanced magnetron sputtering. DLE in pure ZnO is contributed by zinc and oxygen vacancies (VZn+VO), as revealed by photoluminescence (PL) spectroscopy, current–voltage (I–V) characteristic measurement, and spectroscopic ellipsometry. The reduction in the number of VZn states is clearly observed upon Ti doping, resulting in the enhancement of green emission from VO. Interestingly, the thin film with a Ti concentration of 1 at. % shows a higher excitonic emission. Furthermore, the temperature dependence of PL spectra shows that the enhanced excitonic emission originates from the donor-bound exciton promoted by the Ti dopant and native VO. This study shows an important role of the defects in controlling the optical and electronic properties of ZnO films for future optoelectronic applications.

Polarization behavior of zinc oxide thin films studied by temperature dependent spectroscopic ellipsometry

We report on the influence of temperature on the polarization behavior of highly oriented ZnO thin film. First, the investigation of crystal structure change is studied, providing supporting information on the macroscopic-scale polarization of the ZnO thin film. Here, the lattice distortion is investigated by using X-ray diffraction. Furthermore, the role of temperature on the polarization behavior of the ZnO thin film is comprehensively studied by using temperature dependent spectroscopic ellipsometry. Here, the temperature dependent dielectric function analysis and electronic excitation models are used to understand the mechanism of polarization. We found an interesting temperature dependence of electronic transition, where the red-shift absorption and exciton-phonon interaction are observed on the system. This interaction is responsible for the increase of polarization response, which is confirmed by dielectric susceptibility spectra. These results provide important understanding for the control of the polarization dependence on the working temperature of ZnO thin film, which is the essential key in the fabrication of switchable optical devices.

Plasmon–exciton interaction and screening of exciton in ZnO-based thin film on bulk Pt as analyzed by spectroscopic ellipsometry

We study plasmon–exciton interaction in ZnO-based thin film on bulk Pt by using high resolution spectroscopic ellipsometry. ZnO films on quartz are used as reference. This study shows the strong electronic interactions between ZnO film and Pt by considering the significant suppression of exciton in ZnO film, in comparison to ZnO film on quartz. We found that plasmon in Pt are responsible to provide transferred electron for electronic blocking of exciton in ZnO film induce by spontaneous recombination from Pt. In the case of Cu doped ZnO film, we confirm screening effects on exciton and a localized interband transition for both systems (ZnO film on Pt and ZnO film on quartz). In Cu-doped ZnO film, electronic blocking of exciton by Pt plasmon is more pronounce rather than screening effect by interband transition. Our results show the importance of plasmon from substrate and doping to modify the optical properties of wide bandgap semiconductor.

Room temperature analysis of dielectric function of ZnO-based thin film on fused quartz substrate

A set of sample consist of pure ZnO and Cu-doped ZnO film were grown on fused-quartz substrates using pulsed laser deposition (PLD) technique. Here, we report room temperature spectroscopic ellipsometry analysis (covering energy range of 0.5 to 6.3 eV) of pure ZnO film and Cu doped ZnO film at 8 in at. %. The thickness of pure ZnO and Cu-doped ZnO film using in this study is about 350 nm. To extract the dielectric function of ZnO thin film, multilayer modeling is performed which takes into account reflections at each interface through Fresnel coefficients. This method based on Drude-Lorentz models that connect with Kramers-Kronig relations. The best fitting of Ψ (amplitude ratio) and Δ (phase difference) taken by SE measurement are obtained reasonably well by mean the universal fitting of three different photon incident angles. The imaginary part of dielectric function (e2) show the broad peak at around 3.3 eV assigned as combination of optical band energy edge with excitonic states. The exitonic states c...

Simulation of charge carriers generation rate of SiGe quantum dot based intermediate band solar cell

In this paper, the influence of the utilization of SiGe quantum dot on carrier generation rate for intermediate band solar cell application will be discussed. The simulation performed to calculate the generation rate which is a function of the effective band gap dan the absorption coefficient value of the material has been done with 3 variations of germanium composition and 4 variations of quantum dot size. The performance of SiGe with 50% of Germanium composition will be compared with pure Silicon and pure Germanium while the variations of quantum dot size used are 1nm, 2nm, 3nm and 4nm. Simulation results show that increasing numbers of Germanium atoms within the material combined with larger size of quantum dot leads to higher generation rate and helps broadening the range of photon energy which can contribute on the creation of electron-hole pair. Adding Ge into Si quantum dot is found to be effective to increase the generation rate of electrons and holes.

Effect of Ta concentration on the refractive index of TiO2:Ta studied by spectroscopic ellipsometry

We have investigated optical properties of Ta-doped TiO2 thin film on LaAlO3 (LAO) substrate using Spectroscopic Ellipsometry (SE) at room temperature. Amplitude ratio Ψ and phase difference L1 between p- and s- polarized light waves are obtained by multiple incident angles measurement (60°, 70°, and 80°) at energy range of 0.5 – 6.5 eV. In order to obtain optical properties for every Ta concentrations (0.01, 0.4, and 5 at. %), multilayer modelling was performed simultaneously by using Drude-Lorentz model. Refractive index and optical dispersion parameters were determined by Wemple-DiDomenico relation. In general, refractive index at zero photon energy n(0) increases by increasing Ta concentration. Furthermore, optical band gap shows a significant increasing due to presence of Ta dopant. In addition, other optical constants are discussed as well.