Efrat Lifshitz

Building devices from colloidal quantum dots

From quantum dot to quantum dot A wide range of materials can now be synthesized into semiconducting quantum dots. Because these materials grow from solutions, there is scope to combine quantum dots into devices by using simple, low-cost manufacturing processes. Kagan et al. review recent progress in tailoring and combining quantum dots to build electronic and optoelectronic devices. Because it is possible to tune the size, shape, and connectivity of each of the quantum dots, there is potential for fabricating electronic materials with properties that are not available in traditional bulk semiconductors. Science, this issue p. 885 BACKGROUND The Information Age was founded on the semiconductor revolution, marked by the growth of high-purity semiconductor single crystals. The resultant design and fabrication of electronic devices exploits our ability to control the concentration, motion, and dynamics of charge carriers in the bulk semiconductor solid state. Our desire to introduce electronics everywhere is fueled by opportunities to create intelligent and enabling devices for the information, communication, consumer product, health, and energy sectors. This demand for ubiquitous electronics is spurring the design of materials that exhibit engineered physical properties and that can enable new fabrication methods for low-cost, large-area, and flexible devices. Semiconductors, which are at the heart of electronics and optoelectronics, come with high demands on chemical purity and structural perfection. Alternatives to silicon technology are expected to combine the electronic and optical properties of inorganic semiconductors (high charge carrier mobility, precise n- and p-type doping, and the ability to engineer the band gap energy) with the benefits of additive device manufacturing: low cost, large area, and the use of solution-based fabrication techniques. Along these lines, colloidal semiconductor quantum dots (QDs), which are nanoscale crystals of analogous bulk semiconductor crystals, offer a powerful platform for device engineers. Colloidal QDs may be tailored in size, shape, and composition and their surfaces functionalized with molecular ligands of diverse chemistry. At the nanoscale (typically 2 to 20 nm), quantum and dielectric confinement effects give rise to the prized size-, shape-, and composition-tunable electronic and optical properties of QDs. Surface ligands enable the stabilization of QDs in the form of colloids, allowing their bottom-up assembly into QD solids. The physical properties of QD solids can be designed by selecting the characteristics of the individual QD building blocks and by controlling the electronic communication between the QDs in the solid state. These QD solids can be engineered with application-specific electronic and optoelectronic properties for the large-area, solution-based assembly of devices. ADVANCES The large surface-to-volume ratio of QDs places a substantial importance on the composition and structure of the surface in defining the physical properties that govern the concentration, motion, and dynamics of excitations and charge carriers in QD solids. Recent studies have shown pathways to passivate uncoordinated atoms at the QD surface that act to trap and scatter charge carriers. Surface atoms, ligands, and ions can serve as dopants to control the electron affinity of QD solids. Surface ligands and surrounding matrices control the barriers to electronic, excitonic, and thermal transport between QDs and between QDs and matrices. New ligand chemistries and matrix materials have been reported that provide freedom to control the dynamics of excitons and charge carriers and to design device interfaces. These advances in engineering the chemical and physical properties of the QD surface have been translated into recent achievements of high-mobility transistors and circuits, high-quantum-yield photodetectors and light-emitting devices, and high-efficiency photovoltaic devices. OUTLOOK The dominant role and dynamic nature of the QD surface, and the strong motive to build novel QD devices, will drive the exploration of new surface chemistries and matrix materials, processes for their assembly and integration with other materials in devices, and measurements and simulations with which to map the relationship between surface chemistry and materials and device properties. Challenges remain to achieve full control over the carrier type, concentration, and mobility in the QD channel and the barriers and traps at device interfaces that limit the gain and speed of QD electronics. Surface chemistries that allow for both long carrier lifetime and high carrier mobility and the freedom to engineer the bandgap and band alignment of QDs and other device layers are needed to exploit physics particular to QDs and to advance device architectures that contribute to improving the performance of QD optoelectronics. The importance of thermal transport in QD solids and their devices is an essential emerging topic that promises to become of greater importance as we develop QD devices. Colloidal quantum dot device architectures. Colloidal quantum dots (center) may be engineered in size, shape, and surface chemistry and deposited from solution to be integrated as thin-film solids in different electronic and optoelectronic devices that modulate and transmit charge and transduce light and electricity. [Figure courtesy of O. Voznyy and F. S. Stinner.] The continued growth of mobile and interactive computing requires devices manufactured with low-cost processes, compatible with large-area and flexible form factors, and with additional functionality. We review recent advances in the design of electronic and optoelectronic devices that use colloidal semiconductor quantum dots (QDs). The properties of materials assembled of QDs may be tailored not only by the atomic composition but also by the size, shape, and surface functionalization of the individual QDs and by the communication among these QDs. The chemical and physical properties of QD surfaces and the interfaces in QD devices are of particular importance, and these enable the solution-based fabrication of low-cost, large-area, flexible, and functional devices. We discuss challenges that must be addressed in the move to solution-processed functional optoelectronic nanomaterials.

Hydrogen-like Wannier–Mott Excitons in Single Crystal of Methylammonium Lead Bromide Perovskite

A thorough investigation of exciton properties in bulk CH3NH3PbBr3 perovskite single crystals was carried out by recording the reflectance, steady-state and transient photoluminescence spectra of submicron volumes across the crystal. The study included an examination of the spectra profiles at various temperatures and laser excitation fluencies. The results resolved the first and second hydrogen-like Wannier-Mott exciton transitions at low temperatures, from which the ground-state excitons binding energy of 15.33 meV and Bohr radius of ∼4.38 nm were derived. Furthermore, the photoluminescence temperature dependence suggested dominance of delayed exciton emission at elevated temperatures, originating from detrapping of carriers from shallow traps or/and from retrapping of electron-hole pairs into exciton states. The study revealed knowledge about several currently controversial issues that have an impact on functionality of perovskite materials in optoelectronic devices.

Rashba Effect in a Single Colloidal CsPbBr3 Perovskite Nanocrystal Detected by Magneto-Optical Measurements

This study depicts the influence of the Rashba effect on the band-edge exciton processes in all-inorganic CsPbBr3 perovskite single colloidal nanocrystal (NC). The study is based on magneto-optical measurements carried out at cryogenic temperatures under various magnetic field strengths in which discrete excitonic transitions were detected by linearly and circularly polarized measurements. Interestingly, the experiments show a nonlinear energy splitting between polarized transitions versus magnetic field strength, indicating a crossover between a Rashba effect (at the lowest fields) to a Zeeman effect at fields above 4 T. We postulate that the Rashba effect emanates from a lattice distortion induced by the Cs+ motion degree of freedom or due to a surface effect in nanoscale NCs. The unusual magneto-optical properties shown here underscore the importance of the Rashba effect in the implementation of such perovskite materials in various optical and spin-based devices.

Evidence in Support of Exciton to Ligand Vibrational Coupling in Colloidal Quantum Dots

The Perspective focuses on the investigation of an unresolved conflict in semiconductor colloidal quantum dots (CQDs) research, concerning the influence of the immediate surrounding on the optical properties of the materials. Todays advanced synthetic colloidal procedures offer formation of a high-quality inorganic crystallite, capped with various organic/inorganic molecular ligands. The Perspective aims to clarify whether exciton recombination processes in CQDs are influenced by the type of crystallite-ligand bonding and, moreover, whether these excitonic processes experience direct coupling to the ligands vibrational modes. Most ligands used have redox characteristics whose functional groups are added on to the CQDs surface via coordination, covalent or ionic bonding. The surface-ligand bonding introduces electronic states either above or below the intraband/interband energy gap, resulting in electronic passivation or in creation of trapping states that affect intraband and interband relaxation processes. Furthermore, crystalline electronic states may have a direct coupling to molecular vibrational states via direct overlap of electronic wave functions or through a long-range energy-transfer process. Also, photoejected carriers resulting from an Auger process or ionization processes may diffuse temporarily onto a ligand site. These scenarios are discussed in the current publication with supporting theoretical and experimental observations.

Optically detected magnetic resonance of thiol-capped CdTe nanocrystals

The optical properties of thiol-stabilized CdTe nanocrystals have been examined. The thiol groups -SR generate a CdS shell at the interface, leading to a CdTe/CdS core—shell structure. The present paper describes our efforts to identify the influence of the CdTe–SR interface on the optical properties of the nanocrystals, utilizing photoluminescence and optically detected magnetic resonance (ODMR) spectroscopy. The photoluminescence spectrum consists of an excitonic peak, overlapped by a broad band at lower energies. The ODMR spectrum, in the spectral regime of the broad band, showed two resonance signals. They are associated with a trapped hole at an anisotropic site of a cadmium vacancy at the Cd–SR interface and an electron in the conduction band.

Quantum Confinement Regimes in CdTe Nanocrystals Probed by Single Dot Spectroscopy : From Strong Confinement to the Bulk Limit

Sufficiently large semiconductor nanocrystals are a useful model system to characterize bulk-like excitons, with the electron and hole bound predominantly by Coulomb interaction. We present optical characterization of excitons in individual giant CdTe nanocrystals with diameters up to 25.5 nm at 4.2 K under varying excitation power and magnetic field strength. We determine values for the biexciton binding energy, diamagnetic shift constant, and Landé g-factor, which approach the bulk values with increasing nanocrystal size.

Optically Detected Spin and Orbit Resonance of Semiconductor Quantum Dots

The luminescence spectrum of CdSe/CdS core-shell nanocrystals contains a dominant exciton hand located at the CdSe core and an additional weak non-excitonic band, associated with trapped carriers. The present paper describes our efforts to identify the influence of CdSe/CdS interfaces on the localization of photogenerated species, utilizing optically detected magnetic spin and spin-orbit resonance spectroscopy (ODMR). The spin resonance (SR) spectrum showed two resonance signals: the first associated with a trapped hole in a symmetric site within the core, and a second, corresponding to trapped electrons in an anisotropic interface defect site. The spin-orbit resonance (SOR) signal data are presented.

Fundamental Properties in Colloidal Quantum Dots

A multidisciplinary approach for the production and characterization of colloidal quantum dots, which show great promise for implementation in modern optoelectronic applications, is described. The approach includes the design and formation of unique core/shell structures with alloy-composed layers between the core and the shell. Such structures eliminate interfacial defects and suppress the Auger process, thus reducing the known fluorescence blinking and endowing the quantum dots with robust chemical and spectral stability. The unique design enables the generation and sustained existence of single and multiple excitons with a defined spin-polarized emission recombination. The studies described herein implement the use of single-dot magneto-optical measurements and optically detected magnetic resonance spectroscopy, for direct identification of interfacial defects and for resolving exciton fine structure. The results are of paramount importance for a fundamental understanding of optical transitions in colloidal quantum dots, with an impact on appropriate materials design for practical applications.