Insights into the Energy Levels of Semiconductor Nanocrystals by a Dopant Approach

Abstract

The energy levels of semiconductor nanocrystals (NCs) are among the key factors controlling the performance of solar cells [1][2][3][4][5][6] and optoelectronics devices. [7][8][9] Therefore, an accurate determination of the conduction band (CB) and valence band (VB) of the NCs is important and a prerequisite in rational design of the building blocks using the NCs. The energy levels of the NCs are acquirable by X-ray absorption, [10] photoelectron, [11][12][13][14] or scanning tunneling spectroscopy.[15] Theoretical predictions on band structures of the NCs have also been reported. [16][17][18][19][20] Recently, cyclic voltammetry (CV) [21][22][23] has been employed to estimate the CB and VB positions of the NCs. These techniques for determining the energy levels of the NCs generally involve sophisticated instruments and tedious sample preparation and/or complicated operation procedures. For example, the CV method requires that the samples are stable during the measurements. Furthermore, these methods depend on charging the NCs during the measurements, which may lead to a shift in their CB and/or VB positions. Thus, determination of the energy band positions of the NCs still remains a challenge.Herein, we developed an approach to determine the energy levels of the NCs by introduction of certain transitionmetal ions (dopants) into the NCs. It is well-known that the energy level of a dopant is independent on size of the host semiconductor. [24][25][26][27][28][29] If the energy level of the doped transition metal ions located in the energy gap of the host semiconductor NCs, band-gap photoluminescence (PL) of the NCs will usually be quenched. [24,26,29] Instead, a PL peak associated with the doped metal ions (Doping PL) appears. As shown in Scheme 1, bottom of the CB (or LUMO) of the NCs can thus be determined based on the doping PL, given the known position of the energy level of the dopant. The band gap (or HOMO-LUMO energy gap) of the NCs can be readily measured by using UV/Vis spectroscopy. Subsequently, the top of the VB (or HOMO) of the NCs is deducible by subtracting the band gap (or HOMO-LUMO energy gap) of the NCs from the energy level of the CB (or LUMO) of the NCs (see Supporting Information for the detailed procedure to calculate the energy levels of a typical sample).InP NCs and Cu doped InP NCs were used as a model system for determination of the energy levels of the NCs. InP NCs with various sizes and a relatively narrow size distribution [30] and Cu-doped InP NCs possessing an efficient doping PL [26,31,32] were synthesized according to reported methods (see the Supporting Information for the detailed synthetic procedures of all of the NCs concerned in this work, and Figure S1 a for the absorption and doping PL spectra of the as-prepared InP and Cu-doped InP NCs).

Heterovalent‐Doping‐Enabled Efficient Dopant Luminescence and Controllable Electronic Impurity Via a New Strategy of Preparing II−VI Nanocrystals

“…Monovalent doping, for example with Ag + and Cu + ions, has been the subject of research by many groups, because it provides not only color‐center impurities, but also extra holes for electronic‐impurity doping . It could be concluded that different synthesis and doping methods usually enabled different kinds of doping level and dopant location in the NC matrix.…”

Robust and Stable Ratiometric Temperature Sensor Based on Zn–In–S Quantum Dots with Intrinsic Dual‐Dopant Ion Emissions

“…Here, the Mn d‐ states are labeled following the reports of refs. The Cu position was determined from the PL spectra of codoped QDs . CB and VB refer to the abbreviations of conduction and valence bands, respectively.…”

Demystifying Complex Quantum Dot Heterostructures Using Photogenerated Charge Carriers

“…15 16 Dilute Cu doping introduces a single atomic-like energy level in between the CB and VB of the host material. The atomic-like nature of this level shields it from the energy variations arising due the band broadening, strain within the lattice and other solid state effects.…”