Rapid Voltage Sensing with Single Nanorods via the Quantum Confined Stark Effect

Growth Control of InP/ZnSe Heterostructured Nanocrystals

The morphology of heterostructured semiconductor nanocrystals (h-NCs) dictates the spatial distribution of charge carriers and their recombination dynamics and/or transport, which are the main performance indicators of photonic applications utilizing h-NCs. The inability to control the morphology of heterovalent III-V/II-VI h-NCs composed of heavy-metal-free elements hinders their practical use. As a case study of III-V/II-VI h-NCs, the growth control of ZnSe epilayers on InP NCs is demonstrated here. The anisotropic morphology in InP/ZnSe h-NCs is attributed to the facet-dependent energy costs for the growth of ZnSe epilayers on different facets of InP NCs, and effective chemical means for controlling the growth rates of ZnSe on different surface planes are demonstrated. Ultimately, this article capitalizes on the controlled morphology of InP/ZnSe h-NCs to expand their photophysical characteristics from stable and pure emission to environment-sensitive one, which will facilitate their use in a variety of photonic applications.

Electric-Field Fluctuations as the Cause of Spectral Instabilities in Colloidal Quantum Dots

Spectral diffusion (SD) represents a substantial obstacle toward implementation of solid-state quantum emitters as a source of indistinguishable photons. By performing high-resolution emission spectroscopy for individual colloidal quantum dots at cryogenic temperatures, we prove the causal link between the quantum-confined Stark effect and SD. Statistically analyzing the wavelength of emitted photons, we show that increasing the sensitivity of the transition energy to an applied electric field results in amplified spectral fluctuations. This relation is quantitatively fit to a straightforward model, indicating the presence of a stochastic electric field on a microscopic scale, whose standard deviation is 9 kV/cm, on average. The current method will enable the study of SD in multiple types of quantum emitters such as solid-state defects or organic lead halide perovskite quantum dots, for which spectral instability is a critical barrier for applications in quantum sensing.

Cation Exchange during the Synthesis of Colloidal Type-II ZnSe-Dot/CdS-Rod Nanocrystals

Cation exchange is known to occur during the synthesis of colloidal semiconductor heteronanoparticles, affecting their band gap and thus altering their optoelectronic properties. It is often neglected, especially when anisotropic heterostructures are discussed. We present a study on the role of cation exchange inevitably occurring during the growth of anisotropic dot-in-rod structures consisting of a spherical ZnSe core enclosed by a rod-shaped CdS shell. The material combination exhibits a type-II band alignment. Two reactions are compared: the shell-growth reaction of CdS on ZnSe and an exchange-only reaction of ZnSe cores to CdSe. Transmission electron microscopy and a comprehensive set of optical spectroscopy data, including linear and time-resolved absorption and fluorescence data, prove that cation exchange from ZnSe to CdSe is the dominant process in the initial stages of the shell-growth reaction. The degree of cation exchange before significant shell growth starts was determined to be about 50%, highlighting the importance of cation exchange during the heteronanostructure growth.

Time for NanoNeuro

The study of electronic properties of materials at the nanoscale has unveiled physical laws and generated materials such as nanoparticles, quantum dots, nanodiamonds, nanoelectrodes, and nanoprobes. Independently, large-scale public and private neuroscience programs have been launched to develop methods to measure and manipulate neural circuits in living animals and humans. Here, we review an upcoming field, NanoNeuro, defined as the intersection of nanoscience and neuroscience, that aims to develop nanoscale methods to record and stimulate neuronal activity. Because of their unique physical properties, nanomaterials have intrinsic advantages as biosensors and actuators, and they may be applicable to humans without the need for genetic modifications. Thus, nanoscience could make major methodological contributions to the future of neuroscience and, more generally, to biomedical sciences.

Probing Permanent Dipole Moments and Removing Exciton Fine Structures in Single Perovskite Nanocrystals by an Electric Field

Single perovskite nanocrystals have emerged as a novel type of semiconductor nanostructure capable of emitting single photons with rich exciton species and fine energy-level structures. Here we focus on single excitons and biexcitons in single perovskite CsPbI_{3} nanocrystals to show, for the first time, how their optical properties are modulated by an external electric field at the cryogenic temperature. The electric field can cause a blueshift in the photoluminescence peak of single excitons, from which the existence of a permanent dipole moment can be deduced. Meanwhile, the fine energy-level structures of single excitons and biexcitons in a single CsPbI_{3} nanocrystal can be simultaneously eliminated, thus preparing a potent platform for the potential generation of polarization-entangled photon pairs.

Recent Progress of Hybrid Optical Probes for Neural Membrane Potential Imaging

The neural membrane potential of nerve cells is the basis of neural activity production, which controls advanced brain activities such as memory, emotion, and learning. In the past decades, optical voltage indicator has emerged as a promising tool to decode neural activities with high-fidelity and excellent spatiotemporal resolution. In particular, the hybrid optical probes can combine the advantageous photophysical properties of different components such as voltage-sensitive molecules, highly fluorescent fluorophores, membrane-targeting tags, and optogenetic materials, thus showing numerous advantages in improving the photoluminescence intensity, voltage sensitivity, photostability, and cell specificity of probes. In this review, the current state-of-the-art hybrid probes are highlighted, that are designed by using fluorescent proteins, organic dyes, and fluorescent nanoprobes as the fluorophores, respectively. Then, the design strategies, voltage-sensing mechanisms and the in vitro and in vivo neural activity imaging applications of the hybrid probes are summarized. Finally, based on the current achievements of voltage imaging studies, the challenges and prospects for design and application of hybrid optical probes in the future are presented.

All-Optical Detection of Neuronal Membrane Depolarization in Live Cells Using Colloidal Quantum Dots

Luminescent semiconductor quantum dots (QDs) have recently been suggested as novel probes for imaging and sensing cell membrane voltages. However, a key bottleneck for their development is a lack of techniques to assess QD responses to voltages generated in the aqueous electrolytic environments typical of biological systems. Even more generally, there have been relatively few efforts to assess the response of QDs to voltage changes in live cells. Here, we develop a platform for monitoring the photoluminescence (PL) response of QDs under AC and DC voltage changes within aqueous ionic environments. We evaluate both traditional CdSe/CdS and more biologically compatible InP/ZnS QDs at a range of ion concentrations to establish their PL/voltage characteristics on chip. Wide-field, few-particle PL measurements with neuronal cells show the QDs can be used to track local voltage changes with greater sensitivity (ΔPL up to twice as large) than state-of-the-art calcium imaging dyes, making them particularly appealing for tracking subthreshold events. Additional physiological observation studies showed that while CdSe/CdS dots have greater PL responses on membrane depolarization, their lower cytotoxicity makes InP/ZnS far more suitable for voltage sensing in living systems. Our results provide a methodology for the rational development of QD voltage sensors and highlight their potential for imaging changes in cell membrane voltage.

Ratiometric widefield imaging with spectrally balanced detection

Ratiometric imaging is an invaluable tool for quantitative microscopy, allowing for robust detection of FRET, anisotropy, and spectral shifts of nano-scale optical probes in response to local physical and chemical variations such as local pH, ion composition, and electric potential. In this paper, we propose and demonstrate a scheme for widefield ratiometric imaging that allows for continuous tuning of the cutoff wavelength between its two spectral channels. This scheme is based on angle-tuning the image splitting dichroic beamsplitter, similar to previous works on tunable interference filters. This configuration allows for ratiometric imaging of spectrally heterogeneous samples, which require spectral tunability of the detection path in order to achieve good spectrally balanced ratiometric detection.

Improved Surface Functionalization and Characterization of Membrane-Targeted Semiconductor Voltage Nanosensors

Type-II ZnSe/CdS voltage-sensing seeded nanorods (vsNRs) were functionalized with α-helical peptides and zwitterionic-decorated lipoic acids (zw-LAs). Specific membrane targeting with high loading efficiency and minimal nonspecific binding was achieved. These vsNRs display quantum yield (QY) modulation as a function of membrane potential (MP) changes, as demonstrated at the ensemble level for (i) vesicles treated with valinomycin and (ii) wild-type HEK cells under alternating buffers with different [K⁺]. ΔF/F of ∼ 1% was achieved.

Quantum-confined stark effect in the ensemble of phase-pure CdSe/CdS quantum dots

Colloidal semiconductor quantum dots (QDs) have recently attracted great attention in electric field sensing via the quantum-confined Stark effect (QCSE), but they suffer from the random local electric field around the charged QDs through the Auger process or defect traps. Here, QCSE in the ensemble of phase-pure wurtzite CdSe/CdS QDs was studied by applying a uniform external electric field. We observed clear field-dependent photoluminescence (PL) and absorption characteristics in thick-shell CdSe/CdS QDs with 11 CdS monolayers (11 MLs) including a pronounced spectral redshift in PL of ∼2.3 nm and absorption of ∼2.1 nm. The time-dependent PL intensity traces implied that the thick-shell QDs were conducive to realize the Stark shift in QD ensembles due to the effective suppression of the main sources of the local field. These findings were in stark contrast to those of moderate-shell (5 MLs) and ultrathick-shell (15 MLs) CdSe/CdS QDs. The measurement value of exciton polarizability was smaller than the theoretical value, which may be influenced by very few exciton traps. Moreover, the amplified stimulated emission also exhibited obvious optical modulations under the electric field with decreased emission intensity and an increased ultrafast lifetime. Finally, the temporal evolution of the multiexciton process in thick-shell CdSe/CdS QDs indicated that the multiexciton state induced a higher energy state near the band edge, which may weaken the QCSE of a single exciton. Therefore, it was demonstrated that efficient field control over the optical properties of these nanomaterials is feasible and this can open up potential applications in field-controlled electro-optic modulators.

Excitation and Emission Transition Dipoles of Type-II Semiconductor Nanorods

The mechanisms of exciton generation and recombination in semiconductor nanocrystals are crucial to the understanding of their photophysics and for their application in nearly all fields. While many studies have been focused on type-I heterojunction nanocrystals, the photophysics of type-II nanorods, where the hole is located in the core and the electron is located in the shell of the nanorod, remain largely unexplored. In this work, by scanning single nanorods through the focal spot of radially and azimuthally polarized laser beams and by comparing the measured excitation patterns with a theoretical model, we determine the dimensionality of the excitation transition dipole of single type-II nanorods. Additionally, by recording defocused patterns of the emission of the same particles, we measure their emission transition dipoles. The combination of these techniques allows us to unambiguously deduce the dimensionality and orientation of both excitation and emission transition dipoles of single type-II semiconductor nanorods. The results show that in contrast to previously studied quantum emitters, the particles possess a 3D degenerate excitation and a fixed linear emission transition dipole.