Joanna S. Wang

Analysis of thermal band gap variations of PbS quantum dots by Fourier transform transmission and emission spectroscopy

Fourier transmission and emission spectroscopy was employed in the range from 5 to 300 K to measure the thermal band gap shift of 4.7 nm PbS quantum dots. The analytical comparison of fits carried out with the expressions of Varshni and Fan revealed limited accuracy of the Varshni fitting parameters. Evidence is presented that transmission spectroscopy in conjunction with the Fan model concurs with the microscopic material features, resulting in a tool to determine intrinsic properties of quantum dots.

Nanoparticle film deposition using a simple and fast centrifuge sedimentation method

Colloidal nanoparticles (NPs) can be deposited uniformly on flat or rough and uneven substrate surfaces employing a standard centrifuge and common solvents. This method is suitable for depositing different types of nanoparticles on a variety of substrates including glass, silicon wafer, aluminum foil, copper sheet, polymer film, plastic, and paper, etc. The thickness of the films can be controlled by the amount of the colloidal nanoparticle solution used in the preparation. The method offers a fast and simple procedure compared to other currently known nanoparticle deposition techniques for studying the optical properties of nanoparticle films.

Fourier spectroscopy on PbS quantum dots

The manuscript summarizes our current research on PbS quantum dots. The emission and transmission features in the temperature range of 5 K - 300 K of 4.7 nm PbS quantum dots were investigated and theoretically analyzed with the Fan model, which is based upon the phonon-electron interaction. The model - although designed for bulk semiconductors - apply for quantum dots with the potential to determine fundamental properties such as the Debye temperature.

Insulating oxide film formation with acid catalyzed hydrolysis of alkoxide precursors in supercritical fluid carbon dioxide

Insulating oxide films can be produced by hydrolysis of metal alkoxide precursors in the presence of an acid catalyst in supercritical fluid carbon dioxide (sc-CO2). Using tetraethylorthosilicate (TEOS) as a precursor and acetic acid (HAc) as a catalyst, uniform SiO2 films can be formed on surfaces of different substrates according to the reaction Si(OCH2CH3)4 + 2H2O → SiO2 + 4CH3CH2OH. The quality of the SiO2 film is controlled by the rate of hydrolysis of TEOS which is determined by the amount of water available in the system. In our sc-CO2 reaction system, water involved in the TEOS hydrolysis is generated by the in situ esterification process CH3COOH + C2H5OH → CH3COOC2H5 + H2O. In the absence of acetic acid, the reaction proceeds very slowly. The acid catalyzed reaction probably involves proton coordination to the oxygen atoms of TEOS molecules that facilitates the hydrolysis. The acid-catalyzed hydrolysis reaction produces dense SiO2 films instead of porous SiO2 films formed by water added hydrolysis of TEOS in sc-CO2. Formation of SiO2 films via hydrolysis in sc-CO2 is more rapid compared to the traditional hydrolysis reaction at room temperature. In general, metal alkoxide hydrolysis reactions carried out in a closed sc-CO2 system is not affected by moisture in air compared with traditional open-air hydrolysis systems. Using sc-CO2 as a reaction medium also eliminates undesirable organic solvents utilized in traditional alkoxide hydrolysis reactions.

Stability studies of lead sulfide colloidal quantum dot films on glass and GaAs substrates

The stability of colloidal PbS quantum dot (QD) films deposited on various substrates including glass and GaAs was studied. Over a period of months, the QD film sample was re-tested after being left unprotected in air under ambient conditions. Despite exposure to 532 nm laser excitation and cooling to cryogenic temperatures, the initial photoluminescence (PL) remained stable between tests. We also retested a set of samples that had remained under ambient conditions for over 2 years. To track potential changes to the QDs over time, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), optical microscopy, UV-Vis-NIR spectrophotometry and atomic force microscopy (AFM) were employed. Evidence points towards oxidation enforced shrinking of the active QD volume causing a blue shift of the absorption and photoluminescence. The presented studies are important for reliability expectations of light emitters based on PbS QDs.

Luminescence studies for energy transfer of lead sulfide QD films

Lead sulfide (PbS) quantum dots (QDs) of different sizes are deposited with supercritical fluid CO2 (sc-CO2) to form laterally uniform PbS quantum dot films on glass substrates as compared to other deposition methods. Fluorescence and photoluminescence (PL) spectra of PbS QDs obtained from these closely packed films prepared by the sc-CO2 method reveal effective Forster resonance energy transfer (FRET) between PbS QDs of two different sizes, while the films composed of three different sizes of PbS QDs show an even more effective FRET from the smallest to the largest particles. The FRET measured by PL is consistent with that measured by fluorescence spectroscopy at room temperature. From the PL studies of PbS QD films containing two different QD sizes, we demonstrate that the occurrence of FRET under cryogenic conditions is more efficient.

Photoluminescence limiting of colloidal PbS quantum dots

The exposure of colloidal 2 nm PbS quantum dots to growing continuous wave laser excitation at 532 nm increases the photoluminescence intensity with the square root of the optical stimulus. The results herein in conjunction with previous findings [B. Ullrich and H. Xi, Opt. Lett. 38, 4698 (2013)] advocate the square root trend to be the general limiting function for photo-carrier transport and emission of optically excited nano-sized materials. We further show that the excitation of one electron-hole pair per quantum dot defines the saturation threshold for photoluminescence intensity and dynamic band filling.