Photoluminescence Spectral Shift Research Articles

Enabling quantum sensing under extreme pressure: Nitrogen-vacancy magnetometry up to 130 GPa

Engineering a layer of nitrogen-vacancy (NV) centers on the tip of a diamond anvil creates a multipurpose quantum sensor array for high-pressure measurements, especially for probing the magnetic and superconducting properties of materials. Expanding this concept above 100 GPa appears to be a substantial challenge. We observe that deviatoric stress on the anvil tip sets a limit at 40--50 GPa for practical magnetic measurements based on the optically detected magnetic resonance (ODMR) of NV centers under pressure. We show that this limit can be circumvented up to at least 130 GPa by machining a micropillar on the anvil tip to create a quasihydrostatic stress environment for the NV centers. Improved hydrostaticity is quantified using the pressure dependence of the diamond Raman shift, the NV ODMR dependence on applied magnetic field, and NV photoluminescence spectral shift. This paves the way for the reliable use of NV microsensing at pressures above 100 GPa.

A Perovskite-Based Paper Microfluidic Sensor for Haloalkane Assays.

Detection of haloalkanes is of great industrial and scientific importance because some haloalkanes are found serious biological and atmospheric issues. The development of a flexible, wearable sensing device for haloalkane assays is highly desired. Here, we develop a paper-based microfluidic sensor to achieve low-cost, high-throughput, and convenient detection of haloalkanes using perovskite nanocrystals as a nanoprobe through anion exchanging. We demonstrate that the CsPbX3 (X = Cl, Br, or I) nanocrystals are selectively and sensitively in response to haloalkanes (CH2Cl2, CH2Br2), and their concentrations can be determined as a function of photoluminescence spectral shifts of perovskite nanocrystals. In particular, an addition of nucleophilic trialkyl phosphines (TOP) or a UV-photon-induced electron transfer from CsPbX3 nanocrystals is responsible for achieving fast sensing of haloalkanes. We further fabricate a paper-based multichannel microfluidic sensor to implement fast colorimetric assays of CH2Cl2 and CH2Br2. We also demonstrate a direct experimental observation on chemical kinetics of anion exchanging in lead-halide perovskite nanocrystals using a slow solvent diffusion strategy. Our studies may offer an opportunity to develop flexible, wearable microfluidic sensors for haloalkane sensing, and advance the in-depth fundamental understanding of the physical origin of anion-exchanged nanocrystals.

Study of patterned GaAsSbN nanowires using sigmoidal model

This study presents the first report on patterned nanowires (NWs) of dilute nitride GaAsSbN on p-Si (111) substrates by self-catalyzed plasma-assisted molecular beam epitaxy. Patterned NW array with GaAsSbN of Sb composition of 3% as a stem provided the best yield of vertical NWs. Large bandgap tuning of ~ 75 meV, as ascertained from 4 K photoluminescence (PL), over a pitch length variation of 200–1200 nm has been demonstrated. Pitch-dependent axial and radial growth rates show a logistic sigmoidal growth trend different from those commonly observed in other patterned non-nitride III–V NWs. The sigmoidal fitting provides further insight into the PL spectral shift arising from differences in Sb and N incorporation from pitch induced variation in secondary fluxes. Results indicate that sigmoidal fitting can be a potent tool for designing patterned NW arrays of optimal pitch length for dilute nitrides and other highly mismatched alloys and heterostructures.

Detection of Halomethanes Using Cesium Lead Halide Perovskite Nanocrystals.

The extensive use of halomethanes (CH3X, X = F, Cl, Br, I) as refrigerants, propellants, and pesticides has drawn serious concern due to their adverse biological and atmospheric impact. However, there are currently no portable rapid and accurate monitoring systems for their detection. This work introduces an approach for the selective and sensitive detection of halomethanes using photoluminescence spectral shifts in cesium lead halide perovskite nanocrystals. Focusing on iodomethane (CH3I) as a model system, it is shown that cesium lead bromide (CsPbBr3) nanocrystals can undergo rapid (<5 s) halide exchange, but only after exposure to oleylamine to induce nucleophilic substitution of the CH3I and release the iodide species. The extent of the halide exchange is directly dependent on the CH3I concentration, with the photoluminescence emission of the CsPbBr3 nanocrystals exhibiting a redshift of more than 150 nm upon the addition of 10 ppmv of CH3I. This represents the widest detection range and the highest sensitivity to the detection of halomethanes using a low-cost and portable approach reported to date. Furthermore, inherent selectivity for halomethanes compared to other organohalide analogues is achieved through the dramatic differences in their alkylation reactivity.

(Invited) Quasi Phase Transition in a Single File of Water Molecules Encapsulated inside (6,5) Carbon Nanotubes

The confinement of water molecules in one-dimensional channels leads to fascinating new properties which are of critical importance for many processes in biology, geology, catalysis and nanofluidics. Single-walled carbon nanotubes (SWCNTs) are one of the most promising model systems to investigate molecules under confinement. They combined atomically-precise diameters that deliver the necessary level of confinement with a weak interaction of the water molecules with their smooth inner walls. Despite significant efforts, theoretical predictions of the behavior and peculiar properties of a single-file chain of water water molecules (e.g. interesting for ultrafast proton transport) have proven to be very hard to validate experimentally. In this work, we present temperature-dependent (4.2K up to room temperature) photoluminescence (PL) spectra of water-filled and empty chirality-sorted (6,5) SWCNTs. Superimposed on a linear temperature-depenent PL spectral shift of the empty tubes, an additional stepwise PL shift of the water-filled SWCNTs is observed at around 150K. With the empty SWCNTs serving as the ideal reference system, we can assign this observation to temperature-induced changes occuring within the single file molecular chain. Supported by new Molecular Dynamics simulations, we obtain the first experimental evidence of a quasi phase transition of the orientation of the molecular dipoles within such a one-dimensional single file chain of water molecules.

Quasiphase Transition in a Single File of Water Molecules Encapsulated in (6,5) Carbon Nanotubes Observed by Temperature-Dependent Photoluminescence Spectroscopy.

Molecules confined inside single-walled carbon nanotubes (SWCNTs) behave quite differently from their bulk analogues. In this Letter we present temperature-dependent (4.2K up to room temperature) photoluminescence (PL) spectra of water-filled and empty single-chirality (6,5) SWCNTs. Superimposed on a linear temperature-dependent PL spectral shift of the empty SWCNTs, an additional stepwise PL spectral shift of the water-filled SWCNTs is observed at ∼150 K. With the empty SWCNTs serving as an ideal reference system, we assign this shift to temperature-induced changes occurring in the single-file chain of water molecules encapsulated in the tubes. Our molecular dynamics simulations further support the occurrence of a quasiphase transition of the orientational order of the water dipoles in the single-file chain.

Thermal sensing with CdTe/CdS/ZnS quantum dots in human umbilical vein endothelial cells.

An experimental methodology is presented to measure the temperature variation in cells with the usage of CdTe/CdS/ZnS core/shell/shell quantum dots as nanothermometers. The photoluminescence spectral shifts from the endocytosed quantum dots were measured and analyzed to show heat generation in the human umbilical vein endothelial cell following Ca2+ stress. Cytotoxicity evaluation has demonstrated the CdTe/CdS/ZnS QDs are biocompatible to cells. The measured data show that the thermal sensibility of the core/shell/shell nanocrystals has been calibrated and has a linear correlation of 0.16 nm °C-1 along with temperature variation. The photoluminescence spectral shift of QD uptake in the cell indicates a thermogenesis of 3.125 °C.

Photoacidic and Photobasic Behavior of Transition Metal Compounds with Carboxylic Acid Group(s).

Excited state proton transfer studies of six Ru polypyridyl compounds with carboxylic acid/carboxylate group(s) revealed that some were photoacids and some were photobases. The compounds [Ru(II)(btfmb)2(LL)](2+), [Ru(II)(dtb)2(LL)](2+), and [Ru(II)(bpy)2(LL)](2+), where bpy is 2,2'-bipyridine, btfmb is 4,4'-(CF3)2-bpy, and dtb is 4,4'-((CH3)3C)2-bpy, and LL is either dcb = 4,4'-(CO2H)2-bpy or mcb = 4-(CO2H),4'-(CO2Et)-2,2'-bpy, were synthesized and characterized. The compounds exhibited intense metal-to-ligand charge-transfer (MLCT) absorption bands in the visible region and room temperature photoluminescence (PL) with long τ > 100 ns excited state lifetimes. The mcb compounds had very similar ground state pKa's of 2.31 ± 0.07, and their characterization enabled accurate determination of the two pKa values for the commonly utilized dcb ligand, pKa1 = 2.1 ± 0.1 and pKa2 = 3.0 ± 0.2. Compounds with the btfmb ligand were photoacidic, and the other compounds were photobasic. Transient absorption spectra indicated that btfmb compounds displayed a [Ru(III)(btfmb(-))L2](2+)* localized excited state and a [Ru(III)(dcb(-))L2](2+)* formulation for all the other excited states. Time dependent PL spectral shifts provided the first kinetic data for excited state proton transfer in a transition metal compound. PL titrations, thermochemical cycles, and kinetic analysis (for the mcb compounds) provided self-consistent pKa* values. The ability to make a single ionizable group photobasic or photoacidic through ligand design was unprecedented and was understood based on the orientation of the lowest-lying MLCT excited state dipole relative to the ligand that contained the carboxylic acid group(s).

Degradation of Methylammonium Lead Iodide Perovskite Structures through Light and Electron Beam Driven Ion Migration.

Organometal halide perovskites show promising features for cost-effective application in photovoltaics. The material instability remains a major obstacle to broad application because of the poorly understood degradation pathways. Here, we apply simultaneous luminescence and electron microscopy on perovskites for the first time, allowing us to monitor in situ morphology evolution and optical properties upon perovskite degradation. Interestingly, morphology, photoluminescence (PL), and cathodoluminescence of perovskite samples evolve differently upon degradation driven by electron beam (e-beam) or by light. A transversal electric current generated by a scanning electron beam leads to dramatic changes in PL and tunes the energy band gaps continuously alongside film thinning. In contrast, light-induced degradation results in material decomposition to scattered particles and shows little PL spectral shifts. The differences in degradation can be ascribed to different electric currents that drive ion migration. Moreover, solution-processed perovskite cuboids show heterogeneity in stability which is likely related to crystallinity and morphology. Our results reveal the essential role of ion migration in perovskite degradation and provide potential avenues to rationally enhance the stability of perovskite materials by reducing ion migration while improving morphology and crystallinity. It is worth noting that even moderate e-beam currents (86 pA) and acceleration voltages (10 kV) readily induce significant perovskite degradation and alter their optical properties. Therefore, attention has to be paid while characterizing such materials using scanning electron microscopy or transmission electron microscopy techniques.

Solid state solvation effect of a donor–acceptor type fluorescent molecule and its application to white organic light-emitting diodes

We investigated the solid state solvation effect (SSSE) of a donor–acceptor (D–A) type green fluorescent emitter, 3,7-bis(benzimidazol-2-yl)-10-methylphenothiazine (BBMP), and fabricated white organic light-emitting diodes (OLEDs) by utilizing SSSE. After having confirmed that BBMP showed a large solvatochromic photoluminescence spectral shift not only in liquid solutions but also in solid solutions, we induced a sky-blue electroluminescence (EL) by doping the green-emitting BBMP in non-polar host materials. The emission spectrum of BBMP blue-shifted as the polarity of host material decreased, and white OLEDs were obtained by integrating an additional orange light-emitting material, Rubrene, into this sky-blue-emitting OLED structure. When 9, 10-di(naphtha-2-yl)anthracene(ADN) was employed as a non-polar host of BBMP, the device exhibited a sky-blue (λpeak,EL = 475 nm) emission by SSSE with evidently improved EL properties: the luminous efficiency, the external quantum efficiency, and the maximum luminance were 10.4 cd/A, 3.5%, and 46,600 cd/m2, respectively. Finally, the white OLED with an optimized structure of [ITO/NPB (20 nm)/ADN:Rubrene (20 nm)/NPB (2 nm)/ADN:BBMP (20 nm)/TPBi (20 nm)/Alq3 (10 nm)/LiF (1 nm)/Al (100 nm)] demonstrated a luminous efficiency of 5.3 cd/A, an external quantum efficiency of 1.3%, and a maximum luminance of 15,200 cd/m2 with CIE coordinates of (0.32, 0.42) at 100 mA/cm2.

Regularly patterned non-polar InGaN/GaN quantum-well nanorod light-emitting diode array.

The growth and process of a regularly patterned nanorod (NR)- light-emitting diode (LED) array with its emission from sidewall non-polar quantum wells (QWs) are demonstrated. A pyramidal un-doped GaN structure is intentionally formed at the NR top for minimizing the current flow through this portion of the NR such that the injection current can be effectively guided to the sidewall m-plane InGaN/GaN QWs for emission excitation by a conformal transparent conductor (GaZnO). The injected current density at a given applied voltage of the NR LED device is similar to that of a planar c-plane or m-plane LED. The blue-shift trend of NR LED output spectrum with increasing injection current is caused by the non-uniform distributions of QW width and indium content along the height on a sidewall. The photoluminescence spectral shift under reversed bias confirms that the emission of the fabricated NR LED comes from non-polar QWs.

Spectroscopic investigation of electrochemically charged individual (6,5) single-walled carbon nanotubes.

Individual single-walled carbon nanotubes (SWNTs) of (6,5) chirality were investigated by means of optical spectroscopy while their charge state was controlled electrochemically. The photoluminescence of the SWNTs was found to be quenched at positive and negative potentials, where the onset and offset varied for each individual SWNT. We propose that differences in the local environment of the individual SWNT lead to a shift of the Fermi energy, resulting in a distribution of the oxidation and reduction potentials. The exciton emission energy was found to correlate with the oxidation and reduction potential. Further proof of a correlation was found by deliberately doping individual SWNTs and monitoring their photoluminescence spectral shift.

Spectroscopic Investigation of Electrochemically Charged Individual Single-Walled Carbon Nanotubes

Individual single walled carbon nanotubes (SWNTs) of (6,5) chirality were investigated by means of optical spectroscopy while their charge state was controlled electro-chemically. The photoluminescence of the SWNTs was found to be quenched at positive and negative potentials, where the onset and offset varied for each individual SWNT. We propose that differences in the local environment of the individual SWNT lead to a shift of the Fermi energy, resulting in a distribution of the oxidation and reduction potentials. The exciton emission energy was found to correlate with the oxidation and reduction potential. Further proof of a correlation was found by deliberately doping individual SWNTs and monitoring their photoluminescence spectral shift.

Relationship of quantum-well potential profile and luminescence of InGaN/GaN heterostructures

Photoluminescence (PL) of a series of InGaN/GaN heterostructures with multiple quantum wells (QW) differing in their thicknesses and potential profiles that were grown by MOCVD on (0001) sapphire substrates was studied. Samples with varying InGaN composition within a QW layer that were obtained by varying the growth temperature demonstrated a considerably smaller PL spectral shift upon varying the excitation level, greater PL efficiency at low excitation levels, and slower radiative recombination times than samples grown at constant temperature. This was caused by a less pronounced quantum-confined Stark effect in QW of variable InGaN composition. It was established that varying the temperature during during InGaN QW layer growth resulted in higher heterogeneity of the InGaN composition, which was manifested in a higher degree of localization of photo-generated carriers.

Photoluminescence properties of InGaAsN films on Ge(001) vicinal substrates

The InGaAsN films lattice-matched to Ge substrates and having an about 1eV bandgap energy have been grown on Ge(001) vicinal substrates by metal organic vapor phase epitaxy (MOVPE) varying the group-V precursor flow rates, and their photoluminescence (PL) properties have been investigated. Rapid thermal annealing (RTA) with suitable conditions has improved the luminescence properties. The PL spectral shifts with temperature and excitation intensity indicate that a larger flow of the group-V precursors enhances the carrier localization due to the potential fluctuations caused by the compositional non-uniformity and other localized states which originate from impurities or crystal defects. The huge amount of 1, 1-dimethylhydrazine (DMHy), which can efficiently incorporate N, may also introduce a significant amount of carbon species which can act as impurities, or may suppress the surface migration of the group-III atoms resulting in compositional non-uniformity and crystal defects.

Quantum Dot Nano Thermometers Reveal Heterogeneous Local Thermogenesis in Living Cells

The local temperature response inside single living cells upon external chemical and physical stimuli was characterized using quantum dots as nano thermometers. The photoluminescence spectral shifts from endocytosed quantum dots were used to map intracellular heat generation in NIH/3T3 cells following Ca(2+) stress and cold shock. The direct observation of inhomogeneous intracellular temperature progression raises interesting new possibilities, including further innovations in nanomaterials for sensing local responses, as well as the concept of subcellular temperature gradient for signaling and regulation in cells.

Silica-coated CdTe Quantum Dots of Unchanged Size with Intense Photoluminescence at Various Wavelengths

A new method for controlling the photoluminescence (PL) wavelength of silica-coated quantum dots (QDs) without changing the sizes of the core QDs is reported. When silica-coated green-emitting CdTe QDs were refluxed in aqueous solution containing Cd2+ and a sulfur-containing surfactant (thioglycolic acid (TGA)), the PL peak wavelength shifted to red with a significant increase in PL efficiency. This spectral shift was attributed to the reduction of the quantum confinement effect by the formation of small CdS-like clusters very close to the QDs in the silica shells. When the silica-coated QDs were refluxed in a solution of higher pH, the extent of this red shift increased. The larger red shift of the PL with pH is attributed to the hydrolysis speed of TGA increasing with pH. There is faster generation of S2− and as a consequence the growth rate of CdS-like clusters is enhanced. When a different sulfur-containing surfactant (thioglycerol (TG)) was used for reflux, the PL spectral shift and enhancement of PL efficiency became very small. This was because TG did not hydrolyze significantly enough to release S2−. These results demonstrate that the reported reflux procedure is a promising method for tuning the PL color of QDs through selection of the solution pH and the surfactant during reflux.

Spectral Switching of Type-II Quantum Dots by Charging

We report photoluminescence (PL) spectral switching of quantum dots (QDs) by chemically controlling transfers of electrons in and out of the type-II heterostructures. When electron charged, CdTe/CdSe (core/shell) QDs show huge PL blue shifts (up to ∼100 nm) whereas PL of CdSe/CdTe (core/shell) QDs red-shifts. We demonstrate reversible spectral switching of type-II QDs by repeated charging and neutralization processes. The PL spectral shifts are due to the interactions between injected spectrator electrons and type-II character excitons. Electron charged CdTe/CdSe QDs show PL blue shifts because of the strong repulsions between the shell-localized excitonic electrons and injected spectrator electrons. It is opposite for CdSe/CdTe QDs, where injected electrons in surface states attract the holes in the shells. We investigate the interactions of spectrator electrons and excitons as varying the dimensions of type-II QDs. We also compare the type-II QDs with type-I QDs such as CdSe, CdTe, and CdTeSe alloyed QD...

Direct observation of time-dependent photoluminescence spectral shift in CdS nanoparticles synthesized in polymer solutions

Direct observation of time-resolved emission spectra (TRESs) of cadmium sulfide nanoparticles in polymer solutions was carried out with picosecond resolution using a streak camera. The TRESs were found to undergo a pronounced time-dependent Stokes shift, eventually coinciding with the steady-state photoluminescence spectra within an approximately 40 ns delay time. Moreover, approximately 90% of the shift was complete within the first 1 ns after excitation, in contrast to the fact that overall photoluminescence involves very long time constants of 10-100 ns. The observed Stokes shift dynamics was very similar in CdS nanoparticles stabilized in two very different types of polymer solutions. Thus the solvent and/or polymeric stabilizer appeared to have a minimal effect on the shift. We propose that the relaxation proceeds through an internal mechanism involving the fast decay of high-energy traps into relatively slow-decaying low-energy traps. Time-dependent photoluminescence anisotropy experiments also revealed an approximately 1 ns decay component appearing only in the higher-energy end of the photoluminescence spectrum. Because this time constant is too short to represent rotational diffusion of the nanometer-sized particles, it was associated with the rapid relaxation of the high-energy trap states.

Scanning photoluminescent spectroscopy of bioconjugated quantum dots

We report on the application of the bio-conjugated quantum dots (QDs) for a “sandwich” enzyme-linked immunosorbent assay (ELISA) cancer testing technique. Quantum dot ELISA detection of the cancer PSA antigen at concentrations as low as 0.01 ng/ml which is ∼50 times lower than the classic “sandwich” ELISA was demonstrated. Scanning photoluminescence (PL) spectroscopy was performed on dried ELISA wells and the results compared with the same QD samples dried on a solid substrate. We confirmed a “blue” up to 37 nm PL spectral shift in a case of QDs conjugated to PSA antibodies. Increasing of the “blue” spectral shift was observed at lower PSA antigen concentrations. The results can be used to improve sensitivity of “sandwich” ELISA cancer antigen detection.