Perspective pubs.acs.org/JPCL
Contribution of Femtosecond Laser Spectroscopy to the Development of Advanced Optoelectronic Nanomaterials Chi-Hung Chuang and Clemens Burda* Center for Chemical Dynamics and Nanomaterials Research, Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States ABSTRACT: Femtosecond laser spectroscopy has now been a powerful technique for over a decade to investigate charge carrier dynamics in nanoscale optoelectronic systems with a temporal resolution of 100 fs (10−13 s) or better. Both transient absorption and time-resolved photoluminescence spectroscopy are now popular spectroscopic techniques, which are well-established and provide direct insight into the charge carrier dynamics of nanomaterials. In this Perspective, we focus mainly on the developments with regard to studies of semiconductor nanostructures. Controlling the charge carrier dynamics, including hot carrier relaxation, trapping, interfacial carrier transfer, carrier multiplication, and recombination, is essential for successful energy conversion or photocatalysis, to name two major optoelectronic applications. We will show how femtosecond laser spectroscopy evolved into techniques that unveil the dynamic charge carrier properties of semiconductor nanomaterials toward heterostructures and complex nanoarchitectures and that femtosecond time-resolved laser spectroscopy can shine light on the path to novel optoelectronic structures and emergent optoelectronic technologies.
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relaxation.6,9 In addition, Zhang studied the photoinduced electron relaxation dynamics in core/shell structured nanoparticles, namely, AgI/Ag2S and AgBr/Ag2S.8 Prof. M. A. ElSayed explored early on the ultrafast electron−hole dynamics in semiconductor nanoparticles.11−16 His research interests focused on varying the size, shape, and composition of nanomaterials and their effect on the relaxation dynamics.11−14 The CdSe nanoparticles were studied with pump−probe and photoluminescence (PL) spectroscopy over the spectral range from the visible to the infrared region (450−5000 nm).15 Investigations in this broad spectral regime mapped out a route of charge carriers above the band edges (hot carriers) and also the relaxation within the band gap (trapped carriers). Another complex system, CdS/HgS/CdS quantum dot (QD)−quantum well (QW) nanoparticles, were investigated in the El-Sayed group. Both electron and hole trapping times in the HgS well were studied and discussed in terms of the CdS/HgS interfacial charge transfer.16 The group of Prof. Lian has focused on the charge carrier transfer, charge separation, and recombination at the molecule−semiconductor nanocrystal interface.17−19 In dyesensitized solar cells, electron transfer across the interface is a fundamental process where transient absorption (TA) spectroscopy is a powerful tool to detect the electronic TA of ionized molecules and the transient signals from charged semiconductor nanoparticles. Bridging molecules, anchoring groups, and the interfacial environment are of central interest to understand the charge carrier injection dynamics from
emiconductor nanocrystals are crystalline particles in the size range of several nanometers. Their size-dependent properties offer an important source for practical applications in optoelectronics, such as imaging, photocatalysis, and photovoltaics.1−4 To properly tailor these nanocrystals to needs, it is necessary to study and control the charge carrier dynamics in these materials. Femtosecond laser techniques, which offer a powerful approach to directly measure the charge carrier dynamics in nanocrystals, have been used extensively over the past 2 decades.
Ultrafast laser techniques, transient absorption and time-resolved photoluminescence spectroscopy, provide direct measurements of the charge carrier dynamics in photoexcited semiconductor nanomaterials. Already in the 1990s, Prof. Jin Z. Zhang was one of the pioneers studying the charge carrier dynamics in chemically synthesized semiconductor nanoparticles.5−10 Being interested in photoinduced electron dynamics at the liquid−solid interface, he suggested that the surface of nanoparticles plays as important of a role in the carrier relaxation as does the particle size. A large amount of surface atoms with dangling bonds introduces a high density of “surface states”. On the basis of femtosecond time-resolved techniques, early research focused on the effects of particle size and surface states on the electronic structure and on pump-power-dependent © 2012 American Chemical Society
Received: March 13, 2012 Accepted: June 28, 2012 Published: June 28, 2012 1921
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Perspective
band gap of the material, electrons and holes are created with excess kinetic energy. In typical semiconductor solar cells, hot charge carriers quickly cool before their total energy can be captured, a process that limits device efficiency.56 If all of the energy of hot carriers could be captured, solar-to-electric power conversion efficiency could be increased, beyond the Schottky− Queisser limit (∼31% for a single-junction solar cell).57 The excess kinetic energy creates charge carriers with effective temperatures that are higher than that of the lattice. Therefore, the system is far away from thermal equilibrium where it would follow a Fermi−Dirac energy distribution. Upon excitation by an ultrashort laser pulse of appropriate wavelength, the QD is promoted from the ground state to an electronically excited state. The excited electrons begin relaxation back to the ground state via carrier−carrier collisions, and the excess energy is distributed between the excited carriers. In the next phase of the electronic relaxation, the hot electrons start to lose energy to the lattice through electron−phonon coupling, and the average electronic energy/temperature decreases accordingly. The electrons eventually cool down so that their temperature matches that of the lattice. Finally, electron−hole recombination leads the system back to the ground-state equilibrium. (illustrated in Figure 2a).49,51,58
sensitizer molecules to semiconductor nanomaterials and vice versa. Regarding the charge carrier injection in solar cells, the group of Prof. Kamat has been studying electron-transfer rates between QDs and TiO2 nanoparticles with varying QD sizes, as well as the length of linker molecules.20−23 By modulating band energies through size control, they studied the photoresponse and photoconversion efficiency of solar cells. In the past decade, multiexcitons in semiconductor nanocrystals have been extensively investigated by Klimov24−29 and Nozik30−32 through ultrafast laser spectroscopy. Their research interests include the signatures of multiexciton dynamics as studied by femtosecond time-resolved TA and PL spectroscopy, such as exciton−exciton interactions, Auger recombination, and carrier multiplication. The concept of multiexciton effects leads to practical technologies, such as lasing and photovoltaics.27,28,33 A recent report on photocurrent enhancement from multiple exciton generation shows an external quantum efficiency exceeding 100% in PbSe QD-based solar cells.34 TA, time-resolved PL, and spin spectroscopy are the most prevailing techniques in the today’s research community in studying the charge carrier dynamics of semiconductor QDs, as illustrated in Figure 1. Transient electronic absorption (TA)
Exploring the hot carrier dynamics in semiconductor nanomaterials could potentially lead to controllable carrier cooling and charge-transfer rates, which could yield devices that efficiently utilize hot carriers. Figure 1. A schematic of the femtosecond pulse excitation that occurs in semiconductor QDs, followed by the most common spectroscopic measurements using TA, time-resolved PL, and spin relaxation.
The experimentally obtained, time-resolved carrier distribution from 2.28 (band edge) to 2.76 eV is plotted as a contour plot in Figure 2b (unpublished results), where the carrier cooling dynamics from highly excited states to the lowest excited (1s(h)−1s(e)) state in the conduction band can be extracted. Taken at low pump intensity, the average number of excitons per QD is less than one. Although only individual carrier pairs were excited per QD, the concept of temperature is appropriate to characterize the energy distribution of the QD ensemble. The TA contour spectrum is determined by predominantly the electrons and can be described by the equation as follows
spectroscopy is known as a powerful approach with time resolution in the femtosecond scale to extract excited-state information from a quantum-confined system. In TA spectroscopy, a short laser pulse is used to create excited carriers, and a second pulse is used to probe the excited-state relaxation.35−39 The charge carrier dynamics are recorded from the change in absorbance as recorded by the probe pulse due to occupation of excited states as a function of the time delay between the two pulses. Complementarily, time-resolved PL spectroscopy is applied to emissive QDs.40−43 The emission resulting from electron−hole recombination is mainly occurring at the band edge, from surface traps, or from intrinsic trap states. Timecorrelated single-photon counting spectroscopy is a common technique to monitor the PL dynamics in the nanosecond regime, and fluorescence upconversion spectroscopy has a temporal resolution of femtoseconds. In addition, spin dynamics in QDs is an emerging field for quantum information processing.44−48 Producing qubits in semiconductor QDs promises to bring important benefits, including scalable computation and integration of fast electronics within photonic circuits. Hot carrier dynamics studied by femtosecond laser techniques have attracted a great deal of interest in the field of semiconductor QDs.49−55 When a semiconductor QD ensemble is electronically excited with energies above the
TA(εi) ∝ Δα(εi) = −αo(εi) × (fe )
where fe is the electron energy distribution function, εi (equivalent to the photon energy hνi) is the energy of the electronic states, and αo is the ground-state absorption coefficient. Therefore, the measurement of the TA spectrum enables the determination of the electronic energy distribution functions. The time-resolved carrier distributions and their fits to Fermi−Dirac distribution functions at different delay times are shown in Figure 2c. The carrier temperatures were obtained by fitting the obtained population spectra with the Fermi− Dirac function multiplied by the density of state function (δ functions used for discrete energy states in three-dimensionconfined QDs on this spectral scale), as shown below: 1922
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Figure 2. The photoexcited carrier distribution in semiconductor CdSe QDs after a 3 eV laser pulse excitation. (a) A sketched time evolution of the electron distribution f(e) versus the energy to be compared to experimental data in panel c. (b) Contour plot of the time-resolved energy distribution measured with 150 fs time resolution in the energy range of 2.28−2.76 eV (450−545 nm) and a time window of 3000 ps. (c) Relative carrier distribution (open circles) fitted with the Fermi−Dirac function (red lines) at different delay times during the first 500 ps. For clarity, each curve is offset by 0.01. (d) Carrier temperatures as a function of delay time after the femtosecond laser pulse excitation. Rapid cooling occurs in the first 10 ps. The inset shows the cooling within the first 2 ps.
N (ε) ∝
∑ εi
=
∑ εi
1 (ε − εf )/ kt
e
+1
1 e(ε − εf )/ kt + 1
both electrons and holes have subpicosecond transfer rates through the interface, but the relaxation rates are extremely different.
δ(ε − εi)G(εi) 2
δ(ε − εi)(A e−(ε − εi)
/2ω2
)
Understanding the charge carrier dynamics in semiconductor heterostructures will pave the way to emergent optoelectronic technologies and devices based on the design of complex functional nanoarchitectures.
where G(εi) is the Gaussian function at the energy level of εi describing the homogeneous and inhomogeneous broadening due to phonon coupling and size/shape distributions in the QD ensemble, respectively. The constant εf is the Fermi level for the electrons in the semiconductor. The carrier temperatures versus delay time, shown in Figure 2d, were obtained by fitting the energy distribution of the excited carriers. The fast apparent temperature drop, with a time constant of
