A method for determining the single charge spin relaxation lifetime of semiconductor nanocrystals

Abstract

The application relates to a method for measuring the spin relaxation lifetime of single charge of semiconductor nanocrystals. The semiconductor nanocrystals mainly serve as light-absorbing materials, organic molecules serve as charge acceptors, and the testing is performed by using a circularly polarized transient absorption spectrum (TA) technology. The semiconductor nanocrystals are selectively excited by circularly polarized light to generate excitons, and the electrons or holes of the semiconductor nanocrystals are quickly transferred away by the charge acceptors before the spin of the charges is completely flipped, so that the excitons are dissociated, and a long-lifetime charge separation state is generated. On this basis, the spin relaxation lifetime of single charge (electrons or holes) of the semiconductor nanocrystals can be measured.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for measuring the spin lifetime of single charge in a semiconductor nanocrystal-organic molecule hybrid system by combining optical means. BACKGROUND

[0002] In recent years, quantum technology has not only become an international hot research direction, but also an important field of international major power technology strategy layout. Semiconductor electron spin is expected to be applied in quantum computing and plays a crucial role in the development of quantum technology. Spintronics is a technology that uses the spin properties of electrons to process and transmit information. Among them, the injection of electron spin, the transmission and relaxation of spin, and the reading of electron spin are the three key steps of electron spin devices. Understanding and studying the microscopic mechanism of spin relaxation in semiconductors is of great significance for the realization of high-performance spin devices.

[0003] Semiconductor nanocrystals have quantum confinement effects and possess atomic-like discrete energy levels, which will weaken some interactions with phonons. It is predicted that they may have relatively long electron spin lifetimes compared to bulk materials. However, researchers have found that as the size of semiconductor materials decreases, the exchange interaction between electrons and holes in semiconductor nanocrystals smaller than the exciton Bohr radius is greatly enhanced. After excitation by circularly polarized light, the electron-hole spin rapidly relaxes due to strong electron-hole exchange, on the order of a few picoseconds, much shorter than the spin relaxation time required for spin devices. Therefore, how to suppress the exchange interaction between electrons and holes in semiconductor nanocrystals and measure the spin relaxation lifetime of single charge has become a difficult point. According to literature reports, there are currently two main methods. One is photochemical doping, which realizes the measurement of single charge spin lifetime by doping electrons or holes; the other is to use a bias method to separate electrons and holes in space, thereby dissociating excitons.

[0004] We synthesized a stable monodisperse semiconductor perovskite CsPbBr3 nanocrystal sample (CsPbBr3 QDs) and used anthraquinone-2-carboxylic acid organic molecules as a charge acceptor. The two were coupled together through a carboxylic acid bond to form a hybrid heterojunction. We selectively excited the CsPbBr3 nanocrystals by circularly polarized light to generate band-edge excitons in the nanocrystals, and then rapidly form long-lived charge-separated states to measure the hole spin lifetime of perovskite CsPbBr3 nanocrystals. The invention provides a new method for measuring the spin lifetime of single charge in semiconductor nanocrystals and provides some help for the development of semiconductor spin dynamics. SUMMARY

[0005] The application aims to provide a method for measuring single charge spin relaxation lifetime based on a semiconductor nanocrystal-organic molecule system, so as to solve the difficulty in measuring single charge spin lifetime of semiconductor nanocrystal materials.

[0006] The semiconductor nanocrystal can be a traditional II-VI or III-V semiconductor nanocrystal (CdSe, InP, etc.), a perovskite nanocrystal, a full-inorganic perovskite nanocrystal (CsPbX3, X = Cl, Br, I), or an organic-inorganic hybrid perovskite nanocrystal (MAPbX3 and FAPbX3, MA = CH3NH3, FA = CH(NH2)2, X = Cl, Br, I).

[0007] The organic molecule can be an electron or hole acceptor capable of undergoing ultrafast charge transfer with the perovskite.

[0008] The semiconductor-organic molecule system is prepared by a method known in the art. Preferably, the semiconductor nanocrystal is a CsPbBr3 perovskite nanocrystal (hereinafter referred to as QD); the preferred organic molecule is anthraquinone-2-carboxylic acid (hereinafter referred to as AQ); and the preferred preparation method is ultrasonic self-assembly, which is simple to prepare and is expected to realize low-cost electron spin devices in the future.

[0009] In order to verify whether the semiconductor nanocrystal-organic molecule system can accurately measure the hole spin lifetime, the application adopts the following verification technical scheme:

[0010] The semiconductor nanocrystal mainly serves as a light-absorbing material to generate excitons, and the organic molecule serves as an electron acceptor. The circularly polarized transient absorption spectroscopy (TA) technology is used for testing. The circularly polarized light is used to excite the semiconductor nanocrystal to generate excitons, and the electron acceptor quickly transfers the electrons away before the hole spin is completely flipped, so as to realize exciton dissociation and generate a long-lifetime charge separation state. On this basis, the hole spin relaxation lifetime of the semiconductor nanocrystal can be measured. BRIEF DESCRIPTION OF DRAWINGS

[0011] Figure 1 (a) Transient absorption spectra of the QD-AQ system measured under pump-probe lasers with the same circular polarization direction; (b) Transient absorption spectra of the QD-AQ system measured under pump-probe lasers with opposite circular polarization directions; (c) Hole spin relaxation dynamics of the QD-AQ system. DETAILED DESCRIPTION

[0012] The application is further illustrated by examples and drawings.

[0013] Example:

[0014] A method for measuring the spin relaxation lifetime of a semiconductor nanocrystal, the preparation method comprising the following steps:

[0015] 0.1 μmol QD (first exciton absorption peak at 470 nm, 0.5 OD light path in 1 mm cuvette) hexane solution (1.5 mL) and about 2 mg of AQ molecules were mixed and placed in an ultrasonic machine for 10 minutes, followed by filtration with a 0.25 μm pore size polytetrafluoroethylene filter to obtain a QD-AQ solution, the steady-state absorption spectrum of which is shown in Figure 1 a. Since the AQ molecules are insoluble in hexane solution, it can be determined that the AQ molecules are bonded to the QD through coordination. We need to use optical detection means to verify whether the obtained QD-AQ system can achieve single-charge spin measurement.

[0016] The QD-AQ n-hexane dispersion (400 μL) was placed in a 1 mm cuvette on the sample holder of the transient absorption spectrometer, and the transient absorption spectrum was tested with circularly polarized excitation light and circularly polarized probe light (wide spectrum of 400 nm to 800 nm). The energy of the circularly polarized excitation light was equal to that of the band edge exciton, and here the circularly polarized excitation light of 473 nm was selected. During the test, the circular polarization direction of the probe light was fixed, and the circular polarization direction of the excitation light was changed, as shown in Figure 1 a, 1b. The test results shown in a and 1b are the transient absorption spectra when the circular polarization directions of the excitation light and the probe light are the same and opposite, respectively. It can be seen from the figure that the spectral shapes are different when the circular polarization combinations of the excitation light and the probe light are the same (σ + σ + ) and opposite (σ + σ - ) during the test, which is determined by the selection rule of electronic optical transition. The former test obtains the spectral change caused by hole flipping, and the dynamics shows a decay process; the latter obtains the spectral change caused by the generation of hole spin in the opposite direction, and the dynamics shows a generation process. The spin relaxation dynamics extracted from the transient absorption spectrum test results is shown in Figure 1 c. The result obtained by subtracting the dynamics of the same direction excitation and probe light from the opposite direction excitation and probe light is the hole relaxation dynamics, as shown in Figure 1 c. We obtained the spin relaxation lifetime of about 50 ps for this sample.

[0017] The application is a method for measuring single-charge spin relaxation lifetime based on a semiconductor nanocrystal-organic molecule charge separation system, wherein a semiconductor nanomaterial is excited by light to generate an electron-hole pair filled in a conduction band and a valence band, respectively, and an organic molecule is used as a charge acceptor to realize rapid electron transfer. Then, a circular polarization transient absorption spectrometer is used to test hole spin relaxation spectrum and dynamics, and then the single-charge spin relaxation lifetime in the semiconductor nanomaterial is obtained. The method realizes the decoupling of excitons, and provides a new idea for the measurement of single-charge spin lifetime.

[0018] The experimental scheme is simple to operate. The preferred scheme is that CsPbBr3 nanocrystals are used as semiconductor materials, and anthraquinone-2-carboxylic acid is used as the preferred electron acceptor; and the hole spin lifetime is accurately obtained by combining transient absorption spectroscopy technology.

[0019] In summary, the semiconductor nanocrystal-organic molecule charge separation system provided by the application can effectively dissociate excitons and realize the measurement of single-charge spin relaxation lifetime. The application provides a new idea for the accurate measurement of single-charge spin lifetime of semiconductor nanocrystals.

Claims

1. A method of determining the single charge spin relaxation lifetime of a semiconductor nanocrystal, characterized by: The determination method is realized based on a semiconductor nanocrystal-organic molecule charge separation system; the system is constructed based on a long-life charge separation state generated after charge transfer between the semiconductor nanocrystal and the organic molecule under light irradiation; The single-charge spin lifetime test is completed based on an optical transition selection rule and by using circularly polarized transient absorption technology; The semiconductor nanocrystal mainly serves as an optical absorption material to generate excitons, and the organic molecule serves as an electron acceptor, which is tested by using circularly polarized transient absorption spectrum technology; the semiconductor nanocrystal is excited by circularly polarized light to generate excitons, and the electron acceptor quickly transfers the electrons away before the hole spin is completely flipped, so that the excitons are dissociated to generate a long-life charge separation state.

2. The method of claim 1, wherein: The semiconductor nanocrystal serves as an optical absorption body; the semiconductor nanocrystal is one or two or more of CsPbX3, MAPbX3, and FAPbX3 nanocrystals, X = Cl, Br, or I, MA = CH3NH3, and FA = CH(NH2)2; the II-VI group nanocrystal is one or two or more of CdTe, CdSe, and CdS nanocrystals; and the III-V group nanocrystal is one or two or more of InP, GaAs, and GaSb nanocrystals.

3. The method of claim 1, wherein: The organic molecule has a reduction-oxidation potential lower than or higher than that of the semiconductor nanocrystal; the charge transfer rate from the semiconductor nanocrystal to the organic molecule is faster than the spin flipping time of the semiconductor nanocrystal exciton, and the two can form a long-life charge separation state.

4. The method of claim 1, wherein: The photon energy of the excitation light of the circularly polarized transient absorption spectrum is the same as the optical band gap energy of the semiconductor material, so as to ensure that the spin polarization injection efficiency of the electron-hole is high enough; meanwhile, the transient signal excited by the excitation light is less than one-tenth of the band edge exciton absorption intensity on the steady-state absorption spectrum, so as to ensure a single-exciton excitation mode, thereby avoiding multiple excitons and affecting the test result of the hole spin lifetime.

5. The method according to any one of claims 1-4, characterized in that: (1) preparing a solution of the semiconductor nanocrystal and the organic molecule: the solvent used is one or two of n-hexane or n-heptane, the concentration of the semiconductor nanocrystal in the solution is controlled to be between 0.3-0.8 OD of the optical path of the first exciton absorption peak absorbance, the use amount of the organic molecule relative to 1-2 mL of the sample is 1 mg -3 mg, the solution is filtered by using a filter membrane with a pore size of 0.22-0.25 µm to obtain the required solution; (2) measuring the spin lifetime: the solution is placed in a 1 mm cuvette on the sample holder of the transient absorption spectrometer, and the transient absorption spectrum is tested by using circularly polarized excitation light and 400 nm to 1600 nm wide spectrum circularly polarized probe light; the circularly polarized direction of the probe light is fixed during the test, and the circularly polarized direction of the excitation light is changed, so as to obtain the transient absorption spectrum when the circularly polarized directions of the excitation light and the probe light are the same and opposite, respectively, and the spin relaxation lifetime of the sample is obtained.

Images