A method for regulating hot electron transport in metallic heterostructures via controllable electroacoustic coupling

Abstract

The present invention relates to a method for regulating hot electron transport in a metal heterostructure through controllable electroacoustic coupling, and belongs to the technical field of hot electron transport dynamics in nanomaterials. The method comprises: preparing a metal heterostructure; utilizing transient reflectance spectroscopy and extracting peak signal dynamics; simulating through a two-temperature equation and fitting the electroacoustic coupling coefficient G as a variable, iteratively obtaining the G value and quantifying the interface thermal conductivity; obtaining the spatial distribution of hot electrons with different time delays through ultrafast microscopy, and determining the hot electron spatiotemporal distribution evolution curve through two-dimensional Gaussian function fitting; adopting a two-component diffusion model fitting to quantitatively describe the hot electron diffusion behavior; comparing the fitting results with the G value to understand the effect of regulating the electroacoustic coupling on the hot electron diffusion. The present invention controls the electroacoustic coupling strength by regulating the physical properties of the buried layer metal, analyzes the relationship between G and hot electron transport, and provides new insights into improving hot electron transport and optimizing the performance of optoelectronic devices based on hot carriers.

Description

Technical Field

[0001] The present invention relates to a method for regulating hot electron transmission in a metal heterostructure through controllable electroacoustic coupling, and belongs to the technical field of nanomaterial hot electron transmission dynamics. Background Art

[0002] Hot electron dynamics and transport in metal heterostructures play a vital role in various advanced optoelectronic applications, including solar cells, photocatalysis, photodetectors, etc. In most optoelectronic devices, excited carriers need to diffuse to the extraction region before decaying. The non-equilibrium transport of hot electrons is usually accompanied by energy dissipation, which greatly affects the efficiency of the device. Energy dissipation mainly occurs through the heat transfer process from electrons to phonon systems, and its intensity is defined by the electroacoustic coupling coefficient G. The relaxation dynamics of hot electrons are highly sensitive to the electroacoustic coupling strength, but the effect of G on the spatiotemporal evolution of hot electrons is still unclear. Compared with other heterojunctions, metal interfaces enhance the control of cross-interface heat transfer, thereby enabling the regulation of electron relaxation dynamics within metal heterostructures. It makes metal heterojunctions a precise way to adjust the effective electroacoustic coupling strength G by changing the physical properties of the buried layer metal. eff Metallic heterostructures allow for in-depth exploration of the effects of G on hot electron dynamics and transport behavior, and are expected to significantly improve hot electron transport behavior through carefully designed electroacoustic coupling.

[0003] Pump-probe spectroscopy enables direct monitoring of temperature changes in the electron and lattice subsystems on ultrafast timescales by detecting the transient reflectivity (ΔR / R) of the probe light. The two-temperature model (TTM) effectively simulates the temperature dynamics in the electron and lattice subsystems and describes the initial rise and rapid decay of the time-resolved ΔR / R response. Ultrafast pump-probe experiments combined with theoretical modeling have elucidated the size- and pump-power-dependent thermal relaxation mechanisms, as well as the electron-phonon thermal coupling mechanism at metal interfaces under highly nonequilibrium conditions. However, these studies have primarily focused on the transient response within a fixed space, inadvertently overlooking the interplay between the spatial diffusion of hot electrons and temporal thermal relaxation. In recent years, ultrafast imaging techniques based on the pump-probe principle have been used to directly observe ultrafast hot electron diffusion and heat transfer processes in metallic materials and two-dimensional transition metal dichalcogenides. Previous studies have detailed the competing mechanisms between hot electron transport and various cooling channels during the spatiotemporal evolution of hot electrons in noble metals, but a complete understanding of the correlation between the strength of the electrophonon coupling and the hot electron diffusion behavior remains challenging. Controlling hot electron transport is crucial for optimizing optoelectronic device performance. Summary of the Invention

[0004] One of the objectives of the present invention is to provide a method for regulating hot electron transport in a metal heterostructure by controllable electroacoustic coupling. The heat transfer across the interface is adjusted by preparing metal heterostructures with different buried layer thicknesses. The hot electron relaxation dynamics are measured by non-micro-area transient reflectance spectroscopy. G is determined by fitting a dual-temperature model to the experimental results of transient reflectivity in the first few ps. eff The study used ultrafast microscopy to monitor the effect of different effective electroacoustic coupling coefficients on hot electron transport. Directly measuring the temporal and spatial effects of electroacoustic coupling strength on hot electron transport is crucial for elucidating the interplay between electroacoustic coupling and hot electron transport. This approach provides a simple and precise method for regulating hot electron transport in metallic heterostructures, and is universally applicable to a wide range of metals.

[0005] A second object of the present invention is to provide a method for regulating hot electron transport through controllable electroacoustic coupling in a metal heterostructure and its application in optimizing the performance of optoelectronic devices based on hot carriers.

[0006] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are adopted:

[0007] A method for regulating hot electron transport in a metal heterostructure by controllable electroacoustic coupling, characterized by comprising the following steps:

[0008] 1) Preparing metallic heterogeneous films of various thicknesses on a quartz substrate;

[0009] 2) Using non-micro-area transient reflectance spectroscopy to measure the aforementioned metallic heterogeneous thin film samples at the same excitation wavelength and excitation flux, obtaining broadband transient reflectance dynamics data corresponding to all film thicknesses; extracting the experimental results dynamics at the peak signal wavelength;

[0010] 3) The transient reflection dynamics are obtained by simulating the two-temperature equation with the electroacoustic coupling coefficient G as a variable; the experimental result dynamics collected in step 2) are fitted with the simulated transient reflection dynamics, that is, by iterating the value of G, the variance between the simulated transient reflection dynamics and the experimental result dynamics is minimized to obtain the effective electroacoustic coupling coefficient G in the metal heterostructure. eff By quantifying the values ​​of interface thermal conductivity, surface metal film thickness and intrinsic electroacoustic coupling thermal conductivity, the effective electroacoustic coupling coefficient G is determined. eff the impact of;

[0011] 4) Fixing the pump beam in the ultrafast microscope at one position, scanning the metal heterostructure sample with the probe beam to obtain spatial distribution images of hot electrons at different time delays, fitting the distribution images with a two-dimensional Gaussian function, and determining the spatiotemporal distribution evolution curve of the hot electrons in the metal heterostructure sample;

[0012] 5) fitting the hot electron spatiotemporal distribution evolution curve obtained in step 4) using a two-component diffusion model to obtain a two-component diffusion model fitting result, which can quantitatively describe the hot electron diffusion behavior in the metal heterostructure;

[0013] 6) Compare the fitting results of the two-component diffusion model of metal heterostructure samples with different bottom thicknesses with the effective electroacoustic coupling coefficient G eff A comparison is made to quantitatively obtain the effect of electroacoustic coupling on the diffusion of hot electrons.

[0014] Preferably, the metal heterostructures of multiple thicknesses are gold / chromium heterostructures of 40 / 20 nm, 40 / 5 nm and 40 / 0 nm.

[0015] Preferably, the excitation wavelength in step 2) is 800 nm and the excitation flux is 7 mJ / cm 2 , the peak signal wavelength is 485nm.

[0016] Preferably, the dual-temperature equation in step 3) simulates transient reflection dynamics by the equation and Calculate the electron temperature and lattice temperature dynamics curves, where C e and C l The thermal conductivity of electrons and phonons is represented by k e and k l Represents, and S represents the laser source term that explains the delayed relaxation of electron distribution. Then, the dynamic curves of electron temperature and lattice temperature with G as a variable are substituted into the relationship between transient reflectivity and electron and lattice temperature ΔR / R=1 / R(aΔT e +bΔT l ), simulating transient reflection dynamics;

[0017] Preferably, in step 3), the influence of the interface thermal conductivity and the metal film thickness on the effective electroacoustic coupling coefficient is quantified by the formula G eff =G Au +g Au-Cr / d Au described, where G Au Refers to the intrinsic electroacoustic coupling coefficient of gold, g Au-Cr Refers to the heat conduction at the metal interface, d Au Refers to the thickness of the gold film.

[0018] Preferably, the pump light wavelength in step 4) is 700 nm and the pump flux is 7 mJ / cm 2 The probe light wavelength is 485 nm and the pump flux is 20 μJ / cm 2 .

[0019] Preferably, the two-dimensional Gaussian function in step 4) is n(x, y, t) is the spatial distribution image as a function of delay time t and position (x, y); t ,y t represents the spatial position of the Gaussian distribution at a specific delay time t, N is the fitting coefficient, and Describes the variance of the Gaussian curve in the x and y directions respectively.

[0020] Preferably, the two-component diffusion model in step 5) is σ e =Gaussian[u e (t)]| t＝0 , σ eq =Gaussian[u eq (t)]| t＝0 and σ sum (t)=Gaussian[u e (t)+u eq (t)] where u e and u eq represent the diffusion components of hot electrons and electron lattice equilibrium states respectively. The parameter τ cooling It represents the time constant of hot electron cooling. e and D eq These correspond to the diffusion rates of hot electrons and electron lattice equilibrium, respectively, and are considered constants. "Gaussian" refers to the Gaussian fitting function used in the analysis.

[0021] The application of the method of regulating hot electron transport through controllable electroacoustic coupling in metallic heterostructures to optimize the performance of hot carrier-based optoelectronic devices.

[0022] Preferably, the hot carrier-based optoelectronic devices include photocatalysis, photodetectors, and the like.

[0023] Beneficial effects:

[0024] This study uses transient reflectance spectroscopy and ultrafast microscopy to analyze the correlation between hot electron diffusion and the effective electroacoustic coupling coefficient in metal heterojunctions. By manipulating the physical properties of the buried metal layer to control the effective electroacoustic coupling coefficient, hot electron transport can be easily tuned. This provides new insights into the rational design of electroacoustic coupling strength in metal heterostructures to improve hot electron transport and optimize the performance of hot-carrier-based optoelectronic devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1Atomic force microscopy images. AFM measurements of the thickness of gold / chromium heterostructures: a) 40 / 20 nm, b) 40 / 5 nm, and c) 40 / 0 nm. The rough boundaries of the gold / chromium heterostructures are due to the tape used to secure the sample in place. The inset is an AFM image of the measured area. The sample thickness nonuniformity was set to ±5%. Scale bar: 5 μm.

[0026] Figure 2 Fitting of transient reflection dynamics and effective electroacoustic coupling coefficient. a, b) at 800 nm and 7 mJ cm -2 Transient reflection measurements of 40 / 0 nm and 40 / 20 nm Au / Cr heterojunction films within a 7000 ps delay time after pump light excitation. c) Normalized transient reflection dynamics and TTM-simulated transient reflection dynamics at 485 nm, as well as the corresponding effective electroacoustic coupling coefficients of 40 / 0 nm (circles) and 40 / 20 nm (squares) Au / Cr heterojunction film samples.

[0027] Figure 3 Fitting of transient reflection dynamics and effective electroacoustic coupling coefficient. a) At 800 nm and 7 mJ cm -2 Transient reflection measurements of a 40 / 5 nm gold / chromium heterojunction film within a 7000 ps delay time after pump excitation. b) Normalized transient reflection dynamics at 485 nm and transient reflection dynamics simulated by TTM, as well as the corresponding effective electroacoustic coupling coefficient of the 40 / 5 nm gold / chromium heterojunction film sample.

[0028] Figure 4 Schematic diagram of the ultrafast microscope setup.

[0029] Figure 5 Images of the spatiotemporal evolution of hot electrons in a gold / chromium heterojunction film sample. a) Schematic diagram of ultrafast microscopy test data. The distribution of hot electrons (dots) can be fitted with a two-dimensional Gaussian fit (surface). The average variance σ of the two-dimensional Gaussian distribution (dashed line) reflects the size of the hot electron distribution. b, c) Planar projections of the spatiotemporal profiles of hot electrons at several key time delay points of the gold / chromium heterojunction film sample, respectively. All graphs are normalized to highlight the diffusion dynamics. σ obtained by two-dimensional Gaussian fitting at the current time delay is shown in the lower right corner of the color map. d) One-dimensional cross-sectional profile along the x direction at y=0 extracted from b and c, the curve represents the Gaussian fit. The left half corresponds to the Au sample and the right half corresponds to the Au / Cr sample. The inset shows the magnified details of the black square area. e) Gaussian variance σ fitted from the spatiotemporal profile 2, plotted as a function of the pump-probe delay time for 40 / 0 nm and 40 / 20 nm gold / chromium heterojunction films. The curves represent the fitting results of the two-component diffusion model using the least squares method. The three different color blocks represent the three diffusion stages, corresponding to hot electron diffusion, electroacoustic thermalization, and equilibrium diffusion. f) Schematic comparison of the spatiotemporal evolution of hot electrons in the gold / chromium heterojunction film samples. The three background colors correspond to the three different stages in e.

[0030] Figure 6 a) Fitting curve and b) corresponding diffusion parameter results of the spatiotemporal evolution curve of hot electrons in gold / chromium heterojunction films.

[0031] Figure 7 G eff and the diffusion parameters extracted from the two-component diffusion model. a) Effective electroacoustic coupling coefficient G eff and interface conductivity g Au-Cr With the change of chromium layer thickness. b) The time constant of hot electron cooling is used as G eff c)Δσ eq 2 and Δσ max 2 G eff function of d)σ eq 2 / σ e 2 and Δσ max 2 / Δσ eq 2 G eff e) Describe σ 2 Schematic diagram of the meaning of the diffusion parameters of the spatiotemporal evolution and ratio, as well as the relationship between the hot electron diffusion behavior and G eff The relationship between values.

[0032] Among them, 1 is a femtosecond laser, 2 is the first beam splitter, 3 is the first optical parametric amplifier, 4 is a delay line, 5 is a two-axis galvanometer, 6 is the second beam splitter, 7 is the second optical parametric amplifier, 8 is an acousto-optic modulator, 9 is the third beam splitter, 10 is an objective lens, 11 is a sample stage and sample, 12 is an optical filter, 13 is a phase-locked amplifier, and 14 is a control platform. DETAILED DESCRIPTION

[0033] The present invention will be further illustrated below with reference to specific examples. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention. After reading the present invention, modifications of various equivalent forms of the present invention made by those skilled in the art all fall within the scope defined by the claims attached to this application.

[0034] Gold / chromium bilayer heterostructure thin film samples with thicknesses of 40 / 20, 40 / 5, and 40 / 0 nm were deposited on a clean quartz substrate by magnetron sputtering. The thickness of the heterostructure samples was confirmed using atomic force microscopy (AFM). Figure 1 During the deposition process, a step profiler is used to ensure that the thickness deviation is less than 5%.

[0035] Transient reflectance spectroscopy was used to test heterostructure thin film samples of all thicknesses. The transient reflectance spectroscopy test system consisted of a Ti: sapphire laser system (800nm, 35fs, 6mJ -1 The instrument consists of a single-chip optical fiber (TOPAS) system with a 100 nm optical fiber and a 1 kHz repetition rate (1 kHz repetition rate), nonlinear frequency mixing, and a Femto-TA100 spectrometer (Time-Tech Spectra). Briefly, the 800 nm output pulses from the regenerative amplifier were split into two beams using a 50% beam splitter. The transmitted beam was incident on a TOPAS optical parametric amplifier (OPA) to generate an 800 nm pump laser pulse. The reflected 800 nm portion was further split, with less than 10% passing through a neutral density filter, and then focused on a 2 mm sapphire window to generate a white light continuum for the probe beam. An aluminum parabolic reflector was used to focus the probe beam onto the sample. After interacting with the sample, the probe beam was collimated, directed to a fiber-coupled spectrometer with a CMOS sensor, and recorded at 1 kHz. An electric delay line controlled the time delay of the pump and probe beams. A 500 Hz synchronous chopper modulated the pump pulses, and two consecutive probe light pulses (with and without pumping) were used to determine the transient reflectivity (ΔR / R) change. All experimental procedures were performed at room temperature. Here, the excitation wavelength is selected as 800 nm, corresponding to the intra-band transition excitation of gold, and the detection continuous white light range is between 400-700 nm.

[0036] In order to quantitatively quantify the heat transfer ability of electrons to the lattice, the transient reflectivity signal results of the transient reflection experiment ( Figure 2 a, b) Effective electron-phonon coupling factor G calculated by the two-temperature equation for the dynamics of electron and lattice temperature eff The choice of a probe light at 485 nm allows for accurate modeling of the transient reflection signal using TTM. Failure to account for the wavelength dependence can lead to large deviations in the estimation of the effective electroacoustic coupling. Probing near the interband transition threshold of gold (502 nm, 2.47 eV) increases the sensitivity to the electron temperature in transient reflection measurements, leading to a more accurate estimate of the effective electroacoustic coupling coefficient. The electron and lattice temperature dynamics can be described by the following two-temperature equation and Among them C e and C l The thermal conductivity of electrons and phonons is represented by ke and k l , and S represents the source term that explains the delayed relaxation of the electron distribution. The electroacoustic coupling constant is responsible for simulating the intensity of heat transfer between the two subsystems. When there is no electron excitation in the d-band of gold, the electron specific heat capacity C in the gold film e and thermal conductivity k e Can be expressed as: C e =γT e , k e =(T e / T l )k eq , γ is the electron heat capacity constant, k eq is the thermal conductivity of electrons at the initial temperature when the electrons are in equilibrium with the lattice. In the calculation, the boundary is considered to be adiabatic.

[0037] The change in electron energy distribution caused by pump light excitation leads to a change in the dielectric constant, which in turn affects the differential reflectivity. The electron temperature dynamics show an initial rise and rapid decay in the first few picoseconds, while the change in lattice temperature is several orders of magnitude smaller than the change in electron temperature. Therefore, the differential reflection signal in this time domain is dominated by the electron temperature change. After this period of time, the electron and lattice temperatures gradually reach equilibrium, the lattice is heated, and the effect of the lattice temperature change on the differential reflectivity signal needs to be considered. The transient reflectivity, electron temperature change value, and lattice temperature change value have a nearly linear relationship near 485nm, and are least affected by non-thermalized electrons. It is reasonable to assume that the transient reflectivity, electron temperature change value, and lattice temperature change value show a simple linear relationship throughout the thermal electron relaxation process, that is, the following equation: ΔR / R=1 / R(aΔT e +bΔT l The value of a is derived from the calculated peak electron temperature and the peak signal of the experimentally measured ΔR / R dynamics, and the value of b can be obtained from the equilibrium temperature of the electrons and the lattice and the ΔR / R response intensity. eff The simulated ΔR / R is fitted to the experimental kinetic results using the least squares fitting method until the minimum variance output G is obtained. eff . Figure 2 c shows the comparison between the transient reflection dynamics measured at 485 nm and that simulated by the two-temperature equation, and the corresponding G for 40 / 0 nm and 40 / 20 nm gold / chromium heterojunction films. eff G measured by 40 / 0nm and 40 / 20nm Au / Cr heterogeneous films eff were 1.48±0.04×10 16 and 7.57±0.20×10 16 W m -3 K -1The same method was used to embed a thinner buried Cr layer 40 / 5nm gold / chromium heterogeneous film G eff Perform fitting, such as Figure 3 The corresponding G eff The value is 5.43±0.17×10 16 W m -3 K -1 G is calculated by taking into account the non-uniformity of metal film thickness and laser power fluctuation. eff In this study, G eff is used to represent the overall effective heat transfer capacity of hot electrons, which also includes heat transfer across interfaces. The transient reflection dynamics of the Au / Cr heterojunction film clearly shows that the underlying Cr layer significantly enhances the relaxation of hot electrons in the surface Au layer, resulting in the fitting of G eff In addition, G eff The presence of heat transfer between the metal film and the quartz substrate is also considered, as the thickness of the gold film is much lower than the ballistic length. Non-equilibrium electrons interacting with the substrate can accelerate cooling through energy dissipation at the interface.

[0038] Then the heat transfer across the interface is eff The effect of the interface conductance g was quantified by directly introducing Au-Cr , represents the direct heat transfer per unit area across the Au / Cr interface. The interfacial conductance acts in parallel with the electroacoustic coupling conductance through the Au. Therefore, the effective coupled conductance G eff d Au Can be achieved through G eff d Au =G Au d Au +g Au-Cr Converted to electroacoustic coupling conductance G Au d Au and interface conductivity g Au-Cr The sum of can be divided by the constant term d Au Then: G eff =G Au +g Au-Cr / d Au , intuitively speaking, as the heat transfer between interfaces increases, G eff This means that the overall heat transfer capacity of hot electrons is enhanced. In addition, the reduction of the surface gold film thickness will enhance the effect of interface conductivity on G eff Changing the physical properties of the buried metal, such as by increasing its thickness or selecting a metal with a larger intrinsic G, leads to enhanced heat transfer across the interface, which in turn leads to a G eff The intrinsic electroacoustic coupling coefficient of gold is set to 1.1×10 16 W m -3 K -1At a pump flux of 7 mJ cm -2 The interface conductivity g of 40 / 0nm, 40 / 5nm and 40 / 20nm gold / chromium heterogeneous films is Au-Cr 0.152±0.018, 1.73±0.11, and 2.59±0.15 GW m, respectively. -2 K -1 The interfacial conductance of the metal / metal interface is one order of magnitude higher than that of the metal / non-metal interface, which is consistent with previous research results.

[0039] The spatiotemporal evolution of hot electrons in gold / chromium heterojunction films was directly visualized by using a self-built ultrafast microscope system. Figure 4 As shown, the laser emitted by the laser 1 (PH1-20, light conversion, 800kHz, 1030nm) is split by the first beam splitter 2 and used as the pump of the first optical parametric amplifier 3 and the second optical parametric amplifier 7 (OPA, TOPAS-Twins, Light Conversion Company), respectively outputting a pump beam (700nm, 1.77eV) and a detection beam (485nm, 2.56eV), respectively. The delay between the pump pulse and the probe pulse is controlled by a delay line 4 (Newport, M-IMS600LM-S). The probe beam then passes through a two-axis galvanometer 5 (Thorlabs, GVS012 / M) to scan the probe beam and acquire a hot electron distribution image. The probe beam then passes through a second beam splitter 6 and a third beam splitter 9 before entering an objective lens 10 (Nikon, MRH08430, 40×, NA = 0.6). The pump beam is modulated at a frequency of 100 kHz using an acousto-optic modulator 8 (Gooch and Housego, AOMO 3080-125). The modulated pump beam is reflected by the third beam splitter and enters the objective lens 9, where it is combined with the probe beam and focused onto the sample stage and sample 11. The reflected light from the sample is then reflected by the third beam splitter 6. The pump beam is filtered by a filter 12, and the probe beam is then collected by an avalanche photodiode 13 (Thorlabs, APD430A / M). The change in the reflectance (ΔR) of the probe light caused by the pump was detected by a lock-in amplifier 14 (Zurich Instruments, HF2LI). Ultrafast microscopy enables precise monitoring of the diffusion of photoexcited electrons with a temporal accuracy of approximately 200 fs and a spatial resolution of approximately 20 nm. This spatial accuracy is determined by a two-dimensional Gaussian fit of the spatial distribution of hot electrons. The Gaussian fit profile obtained from the spatial mapping captures the evolution of the hot electron distribution in the film, as shown in Figure 2. Figure 5 As shown in a. Figure 5 b, c show the hot electron distribution at three critical pump-probe time delays. The average variance (σ) obtained from a two-dimensional Gaussian fit is used to describe the hot electron distribution at the current time delay.

[0040] exist Figure 6 a shows the square of the mean square error (σ) fitted from the spatiotemporal profiles. 2 ) and plotted as a function of the delay time between the pump and probe beams, with the initial points aligned for comparison. It was observed that the hot electrons in both samples exhibited an evolution characterized by an initial diffusion followed by a contraction. The unusual dip observed in the spatiotemporal distribution of the hot electrons can be attributed to the competitive interaction between the electron and lattice subsystems. Driven by the temperature difference between the two subsystems, the hot electrons transfer heat to the cold lattice, shifting the hot electron diffusion mechanism from hot-electron-dominated diffusion to diffusion dominated by electroacoustic equilibrium. Notably, the onset time of the thermal equilibrium diffusion phase in the 40 / 20nm gold / chromium heterostructure was reduced to approximately 1.7ps from 3.8ps in the 40 / 0nm structure. Due to heat transfer across the gold / chromium interface, the diffusion length in the gold / chromium heterostructure was reduced to almost half. The diffusion rates for the hot-electron-dominated and electroacoustic equilibrium-dominated phases were nearly identical in the gold / chromium heterostructures with different thicknesses. This indicates that alterations in interfacial heat transport do not affect the hot electron diffusion rate in the surface metal. However, these changes do affect the diffusivity of the hot electrons. Furthermore, the reduction in diffusion length represents a reduction in the area heated by hot electrons. This reduction is closely related to the efficiency of photovoltaic devices, as hot electrons need to diffuse to the extraction region to be extracted.

[0041] A two-component diffusion model is used to quantitatively describe the spatiotemporal evolution of hot electrons. By fitting the spatiotemporal distribution evolution curve using the two-component diffusion model with the spatiotemporal evolution parameters as variables using the least squares method, the two-component diffusion model is sufficient to accurately capture the spatiotemporal evolution of two coupled diffusion states with different diffusivities. The formula and initial conditions are as follows: σ e =Gaussian[u e (t)]| t＝0 , σ eq =Gaussian[u eq (t)]| t＝0 and σ sum (t)=Gaussian[u e (t)+u eq (t)], where u e and u eq represent the diffusion components of hot electrons and electron lattice equilibrium states respectively. The time-space evolution parameter τ cooling represents the time constant of hot electron cooling. D e and D eqThe diffusion rates of hot electrons and electron lattice equilibrium are respectively considered as constants. "Gaussian" refers to the Gaussian fitting function used in the analysis. The fitting curve of the gold / chromium heterojunction film is shown in Figure 6 As shown in a. Figure 6 b represents the σ extracted using the two-component diffusion model for Au / Cr heterogeneous films with thicknesses of 40 / 0 nm, 40 / 5 nm, and 40 / 20 nm. e , σ eq and σ max As the value of the time-space evolution parameter. Specifically, σ e represents the initial distribution of hot electrons, σ eq represents the distribution of the electronic lattice equilibrium state after complete electroacoustic thermalization, σ max Represents the maximum diffusion length of hot electrons.

[0042] In thick buried chromium layer heterogeneous films, G eff The increase in indicates enhanced heat transfer from hot electrons to the lattice subsystem. Figure 7 a shows the effective electroacoustic coupling factor G eff It is observed that reducing the thickness of the Cr layer leads to a decrease in the overall electron-to-phonon heat transfer intensity, reducing it to a smaller G eff ( Figure 7 a). Figure 7 a also shows the interface conductivity of the three films g Au-Cr , indicating that the heat transfer at the interface increases with the increase of Cr layer thickness. In the following discussion, the experimental and fitting results of 40 / 0nm and 40 / 20nm Au / Cr heterogeneous films are mainly considered for comparison. eff When the electroacoustic thermalization time τ cooling time increases by 4.1 times, it decreases by 49%, indicating that the ability of electrons to transfer heat to the lattice is enhanced ( Figure 7 b). At the same time, the enhanced electroacoustic coupling strength limits the diffusion ability of electrons and leads to the maximum hot electron diffusion length Δσ max 2 ) decreased by 48%, such as Figure 7 c. In addition, the reduction in the hot electron diffusion length leads to a reduction in the lattice heating area by hot electrons to 26%, as shown by the shrinkage equilibrium distribution Δσ eq 2 As shown ( Figure 7 c) It is crucial to understand the spatiotemporal profile of hot electrons from the perspective of the competing diffusion mechanisms of hot electron diffusion and electroacoustic coupling. Therefore, different electroacoustic coupling intensities can be compared based on the hot electron diffusion behavior. Since the hot electron diffusion behavior extends the heated lattice region beyond the initial energy injection region, the ratio σ is used. eq 2 / σ e2 To describe the diffusion ability of hot electrons. eff The increase of G eff When it is smaller, the diffusion performance of hot electrons is better, such as Figure 7 d. In addition, the dip in the spatiotemporal distribution profile is caused by the electroacoustic coupling process, and the relative amplitude of the dip describes the heat transfer capacity from hot electrons to the lattice. Therefore, Δσ max 2 / Δσ eq 2 Can be used to estimate the electroacoustic coupling strength. max 2 / Δσ eq 2 A higher ratio indicates a more efficient heat transfer from electrons to the lattice subsystem, which is consistent with a larger G eff and stronger electroacoustic coupling strength, e.g. Figure 7 d. In addition, the fitting results of the spatiotemporal profile of the 40 / 5nm Au / Cr film were also analyzed, which was consistent with the above trend. eff The increase in G is associated with the enhancement of the interfacial conductance, which in turn leads to a decrease in the spatiotemporal distribution of hot electrons, manifested as a decrease in the hot electron diffusion behavior, G eff The mutual influence trend between the diffusion of electrons and hot electrons is extremely universal.

[0043] In summary, the parameterized fitting results of the spatiotemporal evolution of hot electrons are consistent with the effective heat transfer intensity G between electrons and phonons. eff By simply changing the physical properties of the underlying metal in the metal heterostructure, such as thickness, etc., the cross-interface heat transfer of the metal interface is affected, and then the effective electroacoustic coupling coefficient in the surface metal is controlled and changed, ultimately achieving the purpose of regulating the thermal electron transport behavior in the surface metal.

[0044] The above specific description further illustrates the purpose, technical solutions and beneficial effects of the invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for regulating hot electron transport in a metal heterostructure by controllable electroacoustic coupling, characterized by: The following steps are involved: 1) Preparing metallic heterogeneous films of various thicknesses on a quartz substrate; 2) Using non-micro-area transient reflectance spectroscopy to measure the aforementioned metallic heterogeneous thin film samples at the same excitation wavelength and excitation flux, obtaining broadband transient reflectance dynamics data corresponding to all film thicknesses; extracting the experimental results dynamics at the peak signal wavelength; 3) The transient reflection dynamics are obtained by simulating the two-temperature equation with the electroacoustic coupling coefficient G as a variable; the experimental result dynamics collected in step 2) are fitted with the simulated transient reflection dynamics, that is, by iterating the value of G, the variance between the simulated transient reflection dynamics and the experimental result dynamics is minimized to obtain the effective electroacoustic coupling coefficient G in the metal heterostructure. eff By quantifying the values ​​of interface thermal conductivity, surface metal film thickness and intrinsic electroacoustic coupling thermal conductivity, the effective electroacoustic coupling coefficient G is determined. eff the impact of; 4) Fixing the pump beam in the ultrafast microscope at one position, scanning the metal heterostructure sample with the probe beam to obtain spatial distribution images of hot electrons at different time delays, fitting the distribution images with a two-dimensional Gaussian function, and determining the spatiotemporal distribution evolution curve of the hot electrons in the metal heterostructure sample; 5) fitting the hot electron spatiotemporal distribution evolution curve obtained in step 4) using a two-component diffusion model to obtain a two-component diffusion model fitting result, which can quantitatively describe the hot electron diffusion behavior in the metal heterostructure; 6) Compare the fitting results of the two-component diffusion model of metal heterostructure samples with different bottom thicknesses with the effective electroacoustic coupling coefficient G eff A comparison is made to quantitatively obtain the effect of electroacoustic coupling on the diffusion of hot electrons.

2. The method for regulating hot electron transport in a metal heterostructure by controllable electroacoustic coupling according to claim 1, characterized in that: The metal heterostructures with multiple thicknesses are gold / chromium heterostructures with thicknesses of 40 / 20 nm, 40 / 5 nm and 40 / 0 nm.

3. The method for regulating hot electron transport in a metal heterostructure by controllable electroacoustic coupling according to claim 1, characterized in that: The excitation wavelength in step 2) is 800 nm and the excitation flux is 7 mJ / cm 2 , the peak signal wavelength is 485nm.

4. The method for regulating hot electron transport in a metal heterostructure by controllable electroacoustic coupling according to claim 1, characterized in that: The pump light wavelength in step 4) is 700 nm and the pump flux is 7 mJ / cm 2 The detection light wavelength is 485nm and the flux is 20μJ / cm 2 .

5. The method for regulating hot electron transport in a metal heterostructure by controllable electroacoustic coupling according to claim 1, characterized in that: In step 3), the dual-temperature equation for simulating transient reflection dynamics is to use the equation and Calculate the electron temperature and lattice temperature dynamics curves, where C e and C l represents the heat capacity of electrons and lattice respectively; the thermal conductivity of electrons and phonons is represented by k e and k l Indicates that, and S represents the laser source term that explains the delayed relaxation of electron distribution; then the dynamic curves of electron temperature and lattice temperature with G as a variable are brought into the relationship between transient reflectivity and electron and lattice temperature ΔR / R=1 / R(aΔT e +bΔT l ), simulating transient reflection dynamics.

6. The method for regulating hot electron transport in a metal heterostructure by controllable electroacoustic coupling according to claim 1, characterized in that: In step 3), the influence of the interface thermal conductivity and the metal film thickness on the effective electroacoustic coupling coefficient is quantified by the formula G eff =G Au +g Au-Cr / d Au Described, where G Au Refers to the intrinsic electroacoustic coupling coefficient of gold, g Au-Cr Refers to the heat conduction at the metal interface, d Au Refers to the thickness of the gold film.

7. The method for regulating hot electron transport in a metal heterostructure by controllable electroacoustic coupling according to claim 1, characterized in that: The two-dimensional Gaussian function in step 4) is n(x, y, t) is the spatial distribution image as a function of delay time t and position (x, y); t ,y t represents the spatial position of the Gaussian distribution at a specific delay time t, N is the fitting coefficient, and Describes the variance of the Gaussian curve in the x and y directions respectively.

8. The method for regulating hot electron transport in a metal heterostructure by controllable electroacoustic coupling according to claim 1, characterized in that: The two-component diffusion model in step 5) is σ e =Gaussian[u e (t)]| t＝0 , σ eq =Gaussian[u eq (t)]| t＝0 and σ sum (t)=Gaussian[u e (t)+u eq (t)] where u e and u eq Represent the component of hot electrons and the electron lattice equilibrium state respectively; the parameter τ cooling represents the time constant of hot electron cooling; D e and D eq correspond to the diffusion rates of hot electrons and electron lattice equilibrium, respectively, and are considered constants; "Gaussian" refers to the Gaussian fitting function used in the analysis.

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