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Quantum Dot Thin-Films as Rugged, High-Performance Photocathodes Nikolay S. Makarov, Jaehoon Lim, Qianglu Lin, John W. Lewellen, Nathan Andrew Moody, Istvan Robel, and Jeffrey M. Pietryga Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b05175 • Publication Date (Web): 03 Mar 2017 Downloaded from http://pubs.acs.org on March 5, 2017

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Quantum Dot Thin-Films as Rugged, HighPerformance Photocathodes Nikolay S. Makarov1, Jaehoon Lim1, Qianglu Lin1, John W. Lewellen2, Nathan A. Moody2, István Robel1*, Jeffrey M. Pietryga1* 1

Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

2

Accelerator Operations and Technology Division, Los Alamos National Laboratory, Los

Alamos, New Mexico 87545, USA * Address correspondence to [email protected]; [email protected]

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ABSTRACT Typical use of colloidal quantum dots (QDs) as bright, tunable phosphors in real applications relies on engineering of their surfaces to suppress the loss of excited carriers to surface trap states or to the surrounding medium. Here, we explore the utility of QDs in an application that actually exploits their propensity toward photoionization, namely as within efficient and robust photocathodes for use in next-generation electron guns. In order to establish the relevance of QDs as photocathodes, we evaluate the efficiency of electron photoemission of QD films of a variety of compositions in a typical electron gun configuration. By quantifying photocurrent as a function of excitation photon energy, excitation intensity and pulse duration, we establish the role of hot electrons in photoemission within the multi-photon excitation regime. We also demonstrate the effect of QD structure and film deposition methods on efficiency, which suggests numerous pathways for further enhancements. Finally, we show that QD photocathodes offer superior efficiencies relative to standard copper cathodes, and are robust against degradation under ambient conditions. TOC GRAPHICS Keywords: Quantum dot, nanocrystal, photoemission, electron gun, photocathode, photoinjector The past three decades of research in colloidal quantum dots (QDs) have witnessed remarkable advances in both our understanding of quantum confinement effects and in our ability to have these effects made manifest in QDs with exquisite control. However, the true technological impact of such progress is just starting to come to fruition, with these advanced properties being employed in macroscale applications. First and foremost, because of their bright, narrow and size tunable photoluminescence (PL), QDs have become the premier choice in downconverting phosphors for ultra-high-definition display technologies by several leading manufacturers, and QD light-emitting diodes (LEDs) are on the verge of commercialization with 2 ACS Paragon Plus Environment

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demonstrations of high luminous intensity1 and internal quantum efficiencies approaching unity.2,

3

Subsequent to the first observations of QD PL,4-6 use as luminophores required

advances in their structural engineering beyond size-tuning of the band gap, most especially the development of effective organic and inorganic (i.e., heterostructuring) passivation schemes to overcome the natural tendency of excited carriers within such a small volume to “ionize” to surface trap states or to the surrounding medium. This is ironic, given that it was facile photoionization that inspired the first studies of semiconductor colloids (for photocatalysis).7-9 That being said, QDs are also actively studied for a range of applications that, as in photocatalysis, require the extraction of photoexcited carriers. These include next-generation photovoltaics10-14 as well as high-performance photodetectors.15, 16 In contrast to light-emitting technologies, these applications actively seek to make use of excited carriers outside of the QD, which typically involves extraction and transport. Accordingly, engineering in this context focuses on optimizing the QD surfaces and QD-QD interface region, seeking to reduce barriers to charge transfer and enhance QD-QD electronic coupling.17-20 When combined with size-tuning of the QD conduction and valence band positions, the result is (ideally) a material that absorbs light and delivers carriers of a fixed potential suited to the needs of the device (i.e., to do electrical work in a solar cell, or to be counted in a detector). In this letter, we exploit the propensity toward photoionization of QDs in a new technological direction, in a demonstration of conductive QD films as efficient, robust photocathodes for potential use in next-generation electron guns, as found in accelerators, X-ray sources and vacuum electronics. A host of vacuum electronic applications require electron beams with increasingly stringent properties, which may include high brightness, high peak- and averagecurrent, low transverse and/or longitudinal emittance (a measure of momentum divergence 3 ACS Paragon Plus Environment

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among the individual electrons within the beam), prompt response, and long shelf- and operational-lifetime.21-24 Satisfying these target parameters motivates an ongoing search and optimization of photoemissive materials, with particular emphasis on controlling emission properties,25,

26

which are typically not independently optimizable in conventional cathode

materials and often dictate the use of materials that are unstable toward even trace amounts of contaminating gases (e.g., oxygen and water vapor).27 QDs are intriguing in this light as chemically robust materials that offer numerous methods for tuning their electronic structures and carrier behaviors28 in ways that can lead to enhanced photoemission.29 In particular, effects of quantum confinement such as the discretization of the electronic density-of-states,

30, 31

and

the relaxation of carrier momentum considerations,32, 33 may offer a novel means for suppressing beam emittance, which in conventional materials is inextricably tied to photoemission quantum yield.34 While, to our knowledge, no studies to date have specifically targeted the use of QDs as photocathodes, in addition to the long-standing photocatalysis work, a few basic investigations of photoionization have implied their potential for such applications. Studies of high-intensity, pulsed excitation of QDs in water have demonstrated that intense femtosecond pulses can yield tens of photoemitted electrons per QD, reportedly by a mechanism involving direct two-photon absorption.35 Photoelectron yield in this study was found to increase with decreasing QD size; a more recent study of QDs in a solvent-free flowing aerosol was able to correlate this dependence with the degree to which the confined electron wavefunction extends beyond the QD boundary (the so-called “evanescent electron wavefunction”),36 which increases as the QD gets smaller. Both of these studies support the role of quantum-confined states in the photoemission process, and demonstrate that even simple engineering (in this case size control) can have a substantial


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impact on photoemission efficiency. In an interesting example of this, building on their previous work in using Mn2+ dopants within QDs to store optical excitation energy to then be used to “upconvert” band-edge carriers in the interest of photocatalysis,37 Dong and coworkers showed that this can also result in photoionization when the total energy exceeds the vacuum level.38 In order to establish the relevance of QDs as photocathodes, we quantify the efficiency of electron photoemission of QD films in a typical electron gun configuration, and investigate the mechanism responsible for electron emission in the multi-photon excitation regime. We explore the cathode photocurrent for a variety of QD compositions (PbSe, CdSe, ZnS, CdSe/CdS core/shell) as a function of the excitation photon energy, intensity, and pulse duration, and demonstrate their superior performance when benchmarked under identical conditions against standard copper cathodes. In contrast to previous studies of solvated or isolated QDs, our setup (Figure 1) consists of a DC electron gun, in which the cathode comprises a solid, solution-cast QD film deposited on a substrate, and operated at −20 kV bias applied to the front face of the QD film through a stainless steel electrode in direct contact with the QDs. The front electrode features a circular 6.4 mm aperture, surrounded by a custom-designed approximately washer-shaped electron lens that provides rough collimation of the emitted electron beam (Figure 1c-d). The photocathode assembly is housed within a vacuum chamber (~10-8 mbar, Figure 1b) with optical access ports that allow for QD films to be excited via front-illumination by 1.55 eV, 3.1 eV, or 4.66 eV photons from a Ti:sapphire amplified femtosecond laser, while the photoemission current is collected by a Faraday cup extending into the chamber and quantified by a picoammeter. We note that one significant difference in studying photoemission from a solid QD film rather than from a flowing QD solution35 or aerosol36 is that the same QDs remain within the 5 ACS Paragon Plus Environment

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excitation volume throughout the experiment. In this scenario, these QDs have the potential to become highly charged if a means for replacing the photoemitted electrons is not provided. Such charges are supplied to the periphery of the excitation volume by the biased electrode at the front of the QD film, but they must travel some distance through the QD film to reach the site of photoemission. Consequently, in these studies, we use electrically conductive films prepared in a manner similar to those used in QD solar cells,10,

11, 39

in which films are deposited by spin-

coating, using a layer-by-layer approach with chemical treatments to remove original surface ligands to enhance charge mobility.


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Figure 1. Photocathode characterization apparatus for QD films. (a) Schematic of the experimental setup for electron photoemission. The QD films deposited on a gold-coated glass substrate are excited from the front at a 20° angle relative to normal, with the fundamental wavelength (800 nm) or its second- (400 nm) or third-harmonic (266 nm) of an amplified femtosecond Ti:sapphire laser through an optical port of the vacuum chamber. The cathode is kept at -20 kV bias, and an electron lens facilitates collimation of the photoelectrons into a Faraday cup where the photocurrent is measured by a Keithley 6517B electrometer. (b) Photograph of sample chamber showing the optical ports (c) side view and (d) front view of the front electrode, showing the aperture and the custom-designed electrode lens. The blue spot in (d) is the excitation laser shining on a fluorescent card during spot-size measurement.

A representative set of photocurrent measurements on NH4I-treated PbSe QD films of ~450 nm thickness (mean QD radius of 2.5 nm, band gap of 0.82 eV) deposited onto gold-coated glass substrates is shown in Figure 2. It is important to note that the valence band edge position of the quantum-confined PbSe nanocrystals in this study is at ~5.0 eV below the vacuum level, 7 ACS Paragon Plus Environment

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therefore, by energy conservation, a single photon in our experimental setup (energy of 4.66 eV or less) cannot directly induce photoemission: consequently, at least 2 to 4 photons must be involved for each electron emission event, depending on excitation wavelength. Surprisingly, excitation of QDs with any of the three photon energies results in appreciable photocurrent, well above that observed from excitation of the bare gold-coated glass (see solid vs. open symbols of corresponding colors in Figure 2). The measured current from gold alone can be considered an overestimation of the potential contribution of the gold substrate to the total photocurrent in PbSe QD cathodes. In reality, when the PbSe QD film is present, no more than 25% of the total excitation power reaches the gold substrate even at 800 nm (corresponding optical density of 0.6) where its transmittance is highest among the three excitation wavelengths (see Supporting Figure S1a for film absorption spectra). Photocurrents as high as 1 nA can be achieved using 800 nm excitation (~100 mW excitation power), whereas higher energy photons can produce up to 2-10 nA of current at 10-50 mW. The experimental trends are highly reproducible, particularly for 400 nm and 266 nm excitation where nearly all photons are absorbed by the PbSe film (see Supporting Figure S1b). To gain better insight into the mechanism of photoemission, it is instructive to replot the photocurrent data in Figure 2 as a function of the average number of photons per laser pulse absorbed by a QD, denoted by . Using values of QD absorption cross sections (σ) at the excitation wavelengths and spot size measurements of the excitation beam (Figure 1d) to determine the pump fluence (j, defined as the number of photons per pulse per unit area), is given by the expression = σ · j. At relatively low pump fluences, corresponding to ≤ 1, the photocurrent (I) increases as a power law of the form I = x (shown as a linear dependence on the logarithmic plot of Figure 3a), where x is expected to represent the number of photons


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required to generate a single electron in the photoemission process. Given that ~5.0 eV of energy is needed to excite a valence band electron to the vacuum level in our PbSe QDs, energy conservation requires 2 photons of either 4.66 eV or 3.1 eV, or 4 photons of 1.55 eV for the generation of each photoelectron (see inset of Figure 3a for energy diagram). These considerations are in excellent agreement with the values extracted from the power law fits (see solid straight lines in Figure 3a), yielding x of 1.8, 2.3, and 4.2 for 4.66 eV, 3.1 eV and 1.55 eV excitation photon energies, respectively. At higher excitation fluences at which > 1, the photocurrent tends to saturate, likely as a result of charge depletion within the excitation volume of the QD film (as discussed in more detail below).

Figure 2. Observation of electron photoemission current from PbSe QD film photocathodes as a function of excitation power at 266 nm (solid black squares), 400 nm (solid blue circles) and 800 nm (solid red triangles) wavelengths. Open symbols with corresponding colors show control experiments of photocurrent on bare goldcoated glass (used as substrates for QD deposition) excited at the three respective wavelengths.


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Although resonant multi-photon absorption by an electron has been invoked to explain similar QD photoemission in previous studies,35, 36 alternative mechanisms should be considered as well, such as Auger ionization40 of multiple photogenerated conduction band electrons within a QD. In an attempt to discern among multiple possible mechanisms, we used pulse-widthdependent photocurrent measurements to estimate the time window within which photoexcitation must occur in order to produce electron emission. In these experiments, we monitor the current from PbSe QD photocathodes excited by 1.55 eV laser excitation as pulse widths (τpulse) are varied over the 50 to 2200 fs range while keeping the average power constant at 90 mW, corresponding to ≈ 2.7 (Figure 3b). In this way, we can probe the operative “lifetime” of the photoemission process and obtain the maximum time window (i.e. the maximum τpulse) of the multiphoton absorption process that still results in significant photocurrent. The observed exponential dependence of the cathode current on τpulse indicates a lifetime in the 0.5 − 0.7 ps range, shown by the hashed shaded area of Figure 3b. This time scale is consistent with the subpicosecond-to-picosecond hot electron cooling via electron-phonon coupling previously measured for PbSe QDs,41 suggesting the involvement of hot carriers that have yet to relax to the band edge. This agrees with our assessment in Figure 3a indicating a 4photon excitation process: whether photoemission occurs via resonant multi-photon absorption or Auger-assisted ionization, energetically, both of these processes require the participation of hot electrons in order to reach (or exceed) the ionization potential with only four 1.55 eV photons. Importantly, simultaneous nonlinear nonresonant 4-photon absorption can be excluded as a potential mechanism on the basis of its expected 1 τ 3pulse dependence (dash-dotted line in Figure 3b; see Supporting Information for details), in clear contrast to measurements. In an attempt to distinguish between the two remaining mechanisms, we estimate their 10 ACS Paragon Plus Environment

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corresponding probabilities, assuming the involvement of four photons, and compare them to our measured photon-to-electron quantum efficiencies. In the case of resonant (via real intermediate states) multi-photon absorption, the first absorbed photon in a QD leads to interband excitation of an electron from the valence band into the conduction band, while the three subsequent photons increase the energy of the electron within the conduction band via intraband absorption. We can estimate the probability of this multi-photon process by knowing the respective absorption cross sections and the pump fluence. The interband absorption cross section of our PbSe QDs at the excitation wavelength is ~1.2×10-15 cm2. In contrast to intraband absorption in bulk semiconductors in which this process is suppressed by crystal momentum conservation, recent studies of PbSe QDs42 have shown that conduction band electrons in these QDs have considerable, essentially wavelength-independent absorption cross sections on the order of 10-17 cm2, i.e., only about two orders of magnitude lower than the interband cross section. Assuming a negligible dependence of the intraband cross section on photon energy and absolute energy of the electron within the conduction band, using Poissonian statistics and the above two cross section values, we estimate the probability of the resonant 4-photon absorption to be ~2×10-6 for a constant pump fluence of 2.3×1015 cm-2, independent of the pulse width (see Supporting Information for details). This value is larger than, and therefore consistent with, our measured photon-to-electron quantum efficiency of ~10-8 for this 4-photon scenario, as other inefficiencies, such as coupling of intrinsic QD electronic states to vacuum states or various losses in charge collection are expected to play a significant role. Auger-assisted ionization is another feasible mechanism to consider, as Auger processes have been universally shown to be the dominant multi-exciton recombination mechanism in monocomponent QDs33, 43 and have been invoked to explain a wide variety of QD phenomena


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such as photodarkening,44 photoluminescence intermittency,45, 46 and high-current efficiency rolloff (so-called “droop”) in QD light-emitting diodes.47 To estimate the probability of Augerassisted ionization under our conditions, we will consider the probability of a single QD absorbing four 1.55 eV photons via interband absorption to generate four “hot” conduction band electrons, in essence a tetraexciton. Next, we estimate the probability that the requisite chain of Auger recombination events, which formally can be considered to proceed through intermediate triexciton and biexciton states before resulting in a single exciton with energy in excess of the ionization potential, can occur all within the 0.5 ps hot-electron lifetime. Within this simplified phenomenological model, we will use the known biexciton, triexciton and tetraexciton lifetimes33, 48 of τ2X = 50 ps, τ3X = 13 ps, and τ4X = 5 ps, respectively, for our PbSe QDs and consider the probability of all these processes occurring within the hot-carrier lifetime. With a pump fluence of 2.3×1015 cm-2 and an interband absorption cross section of ~1.2×10-15 cm2, the Poissonian tetraexciton generation probability is ~0.15. When multiplied by the probability of tetraexciton, triexciton, and biexciton decay within the 0.5 ps time window, we obtain a probability of ~4×10-6, a value very similar to the probability of resonant 4-photon absorption derived above, with a similar expected pulse-width dependence based on hot-electron cooling rates. Therefore, both mechanisms seem feasible and distinguishing between them would require a more detailed model and thorough investigations of intraband relaxation in QDs as a function of carrier excess energy, which are beyond the scope of the current work. Regardless of the mechanism, however, the reliance on hot carriers ensures a rapid cathode response time comparable to intraband carrier cooling lifetimes, and therefore in the sub-ps range. Our present setup does not allow for a direct response-time measurement, but the fact that the photocurrent essentially vanishes for pulse widths above 2 ps verifies this as an approximate


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upper bound for the duration of the photoemission process in an individual QD. This is in contrast to the previously observed Mn2+-dopant mediated photoionization, which instead involves much longer-lived band-edge (ca. 20 ns lifetime) and dopant (ca. 5 ms lifetime) carriers,38 and can therefore be many orders of magnitude slower. In either case, it should be mentioned that the individual QD process duration is only a starting point in determining ultimate promptness. On the macroscale, the promptness of a QD film will depend on other factors as well, such as film thickness, surface roughness, absorption depth of photons, and various elastic and inelastic electron scattering processes.

Figure 3. Probing the photoemission mechanism in PbSe QD films. (a) Scaling of the photocurrent as a function of the average number of photons absorbed per PbSe QD per excitation pulse () for 266 nm (black squares), 400


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nm (blue circles) and 800 nm (red triangles) wavelengths. The slopes of the fits (solid lines) in the low-excitation regime indicate the number of absorbed photons per photoelectron generated. Inset: Schematic energy diagram (to scale) showing the number of photons of different wavelengths (same wavelength color scheme) required to excite an electron from the valence band to the vacuum level. This expected number of absorbed photons per emitted photoelectron is in good agreement with the experimental values extracted from the slope of the fits, confirming our assumptions. (b) Photocurrent as a function of excitation pulse width at constant per-pulse energy using 800 nm photons. The experimental data show an exponential dependence of current on inverse pulse width, with lifetimes in the 0.5 – 0.7 ps range, consistent with hot-carrier intraband relaxation times. The pulse-width dependence of the photocurrent clearly deviates from the one expected for simultaneous nonresonant four-photon-absorption (expected dependence indicated by the dash-dotted line).

One of the main advantages of using QDs for cathode applications is the wide variety of material compositions available, including multi-component heterostructures whose electronic structure can be tailored to match design-specific needs.28 To illustrate some of these possibilities, we synthesize CdSe/CdS core/shell heterostructures and compare their performance as photocathodes with conventional monocomponent CdSe QDs. As recent QD-size-dependent photoelectron spectroscopy has shown,36 the relative photoelectron yield upon ultraviolet excitation is higher for small QDs, explained by the authors as a consequence of the higher proportion of the electron wavefunction extending outside of the QDs as particle size is reduced. Here, we take this approach a step further and design CdSe/CdS heterostructures featuring a quasi-type-II band alignment that promotes the spatial separation of electrons from holes via reduced wavefunction overlap.49 Because of reduced electron-hole Coulomb interaction, removing electrons from these core/shell structures should be more efficient than in the case of core-only CdSe QDs. Figure 4 compares the photocurrent using 4.66 eV excitation of photocathodes made of CdSe (blue) and CdSe/CdS (red) QDs, respectively. As the top of the valence band of these QDs is ~6.7 eV below the vacuum level, two 4.66 eV photons are required for the generation of each photoelectron. The maximum photocurrent is boosted more than threefold to 6.9 nA upon heterostructuring compared to core-only CdSe QDs (Figure 4) at excitation powers of 7.7 – 7.9 mW, showing the utility of this approach in optimizing cathode performance. More sophisticated approaches, such as the use of aligned films of type-II or quasi-type-II dot-in14 ACS Paragon Plus Environment

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rod nanoheterostructures50, 51 or “Janus”-type heterostructures52, 53 can potentially further boost cathode performance.

Figure 4. Wavefunction engineering in CdSe/CdS heterostructured QDs (red circles) leads to a significant increase (more than 3-fold at maximum excitation intensity) in photocurrent compared to conventional monocomponent CdSe QDs (blue squares) of the same band gap under 266 nm excitation.

Finally, to quantify the efficiency of QD-based photocathodes, we plot the number of electrons vs. the number of photons per laser pulse under identical conditions (10-8 mbar, 4.66 eV excitation, -20 kV acceleration voltage) for four different QD material compositions: PbSe, CdSe, CdSe/CdS, and ZnS (Figure 5). We have also plotted the efficiency of a standard Cu cathode after cleaning in vacuo by UV laser exposure, under the same conditions. Promisingly, all of our QD samples exhibited maximum quantum efficiencies exceeding that of the polished bulk copper electrode, with the CdSe/CdS system showing the highest value of 4 × 10-6. Although this is several orders of magnitude lower than the efficiency of the best performing bulk semiconductor thin-film cathodes, such as K2CsSb54 or “negative electron affinity” (NEA) GaAs,55 the QD photocathodes were handled in air for loading in the chamber. This feat would


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be quite impossible for either of the bulk semiconductor cathodes, whose efficiencies degrade dramatically over 10s of hours even under 10-10 mbar vacuum conditions54, 55 due to trace gas contamination and stray ion bombardment. Further, although exposure to very low levels of water vapor (on the order of 10-12 torr partial pressure) during fabrication is known to enhance the efficiency of K2CsSb photocathodes, exposure to the amount of water vapor even in the high desert air of Los Alamos, NM, (ca. 1 torr) would kill the photocathode instantaneously.27 This sensitivity is generally accepted as a universal limitation of bulk semiconductor photocathodes. In contrast, the QD films showed no deterioration in performance after several weeks in the 10-8 mbar vacuum chamber, and even storage in air for two months resulted in only a 55% drop in performance. The data in Figure 5 reveal that the quantum efficiency of QD films increases with photoexcitation intensity in the low-power regime (the slope of the curves is higher than unity in that region, see e.g. the dashed line as a guide corresponding to linear dependence of photocurrent vs. photon intensity and thus a constant quantum efficiency) and reach saturation at high excitation powers, where quantum efficiency becomes constant or declines slightly. More complete studies of this phenomenon are planned and were limited herein by the power of our light source (which determined the upper limit of power for which current was measured in each case) and applied electric field (which limited the charge per bunch that could be extracted, as discussed in the next paragraph). Nonetheless, the highest extracted steady-state current of ~10 nA (Figure 2) corresponds to on the order of 10 pC per bunch, or (assuming a pulsewidth of 2 ps) a peak current of 160 A/cm2, on a par with the requirements of prominent XFEL56-58 experiments, and even exceeding that of emerging applications such as Ultrafast Electron Diffraction (UED).59, 60


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The “roll-off” in emitted current as a function of laser power can be due to space-charge effects, which can effectively prevent photoionized electrons from being emitted from the film, if extracted charge per bunch is high enough. Analysis of space charge fields according to the manner of Ref. 61 shows that this effect should indeed dominate at high excitation laser power. With emission time on the order of 2 ps, the electric field associated with a 10 pC bunch charge is 0.35 MV/m, which is more than three times the applied extraction field of 0.1 MV/m. Under our conditions, the onset of space charge limits, in fact, occurs when the bunch charge approaches 2-3 pC. Importantly, space-charge effects will manifest for any cathode composition; elimination will require modification of the experimental apparatus (i.e., higher accelerating voltage and/or shorter collection distance). Beyond space charge effects, however, experimental evidence suggests that electron depletion is also a current-limiting factor. First, measurements of a cathode made of the same CdSe QDs as shown in Figure 4, but without chemical treatments to remove the original passivating ligands during deposition, showed unstable behavior. Specifically, at moderate to high excitation power, current was observed to diminish with continuous exposure to the laser over a period of seconds, typically stabilizing at a value up to an order of magnitude lower than the initial value. The initial value was recovered (and the decline was repeated) upon measurement after several minutes without illumination, which rules out the possibility that damage to the film is responsible. Second, QD films deposited directly onto quartz without a layer of gold show either much more dramatic current saturation effects (for high photon energies where excitation depth is shallower, Supporting Figure S2, squares), or overall lower current (for lower photon energies, Supporting Figure S2, circles). This suggests that the presence of the thin gold layer aids in replenishing emitted electrons, despite not actually being


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in direct contact with the front electrode, likely by shortening the distance electrons must travel through the QD film (from mm, laterally from the front electrode aperture edge to the excitation spot, down to merely 100s of nm through the thickness of the film). Other effects, such as the metal film altering the carrier density distribution in the QD film itself through formation of a junction at the metal-semiconductor interface,62, 63 are possibly operative as well. According to field-effect transistor measurements (see “Methods and Materials” for details), QD films prepared in a manner identical to the most conductive films for which photoemission was measured exhibit carrier mobilities on the order of 10-5 cm2V-1s-1, independent of QD composition, many orders of magnitude lower than typical bulk semiconductors. Therefore, it is very likely that all photocurrents, and hence all quantum efficiencies, measured under our conditions (intense pulsed excitation, DC collection and measurement) are negatively impacted by electron depletion effects.


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Figure 5. Photoemission quantum efficiency of CdSe/CdS (red circles), PbSe (blue triangles), ZnS (green diamonds) and CdSe (orange stars) QDs in comparison to bulk copper (black squares) under identical conditions. The red dashed line is a guide to the eye showing a linear photocurrent-vs-excitation-photon dependence (unity slope) of photoelectrons/pulse on excitation photons/pulse corresponding to a constant quantum efficiency of 4×10-6, the peak value for CdSe/CdS QDs. The quantum efficiency of QDs increases linearly at low intensities (photocurrent is quadratic on excitation intensity at 266 nm excitation, as discussed in Fig. 3a) and it reaches a constant saturation value at high intensities for CdSe/CdS and CdSe QDs (a slope close to unity), while it declines slightly for PbSe and ZnS QDs (a slope less than unity).

In conclusion, we have established the relevance of a new class of solution-processed photocathode materials based on colloidal QDs. These new photocathodes demonstrate environmental stability far beyond bulk semiconductor photocathodes, and photoemission quantum efficiencies exceeding that of a polished copper photocathode in a side-by-side comparison. This is a very promising result, especially considering the several yet unexploited pathways for further optimization of both the materials themselves, as well as of the excitation and current collection methods. Given the role of hot electrons in the photoemission process, one approach, already explored in next-generation QD-photovoltaics,48 is to choose or design QDs 19 ACS Paragon Plus Environment

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with slow hot-electron cooling rates. Further, if Auger ionization is the dominant mechanism, then employing QDs with enhanced Coulomb interactions64 is another strategy that could lead to improvements. Another simple method for enhancement may be to make the most efficient use of excitation photons by using QDs with ionization potential (i.e., valence band position) that is an integer multiple of the excitation photon energy, which can be accomplished by careful choice of QD composition and size. Correspondingly, excitation energy can also be tuned, and it is even possible to use laser pulses further in the ultraviolet energy spectrum that match the ionization potential and therefore reduce the order of the photoionization process to a single photon; although this is less practical in the context of eventual applications in real electron guns, it would offer the opportunity to probe the fundamentals of photoemission in the single-photon regime. Another immediately practical pathway is to explore QD films treatments that produce higher carrier mobilities.20, 65, 66 Finally, the dependence of photocurrent on the structure of the QD (CdSe vs. CdSe/CdS) suggests the potential of structures with reduced electron-hole interaction strength, including the previously mentioned type-II heterostrucutres, or of shapecontrolled nanoparticles that may feature enhanced “leakage” of the evanescent electron wavefunction in particular directions. Materials and Methods General considerations. Cadmium oxide (CdO, 99.99+%), selenium (200 mesh, 99.999%), zinc acetate (Zn(ac)2, 99.99%), 1-dodecanethiol (DDT, 98%), oleic acid (OA, 90%), 1octadecene (ODE, 90%), tri-n-octylphosphine (TOP, 97%), lead bromide (PbBr2, 99.999%), selenium shot (Se, 99.999%) and 2,6-difluoropyridine (DFP, 98+%) were purchased from Alfa Aesar. n-trioctylamine (TOA, 98%), di-n-isobutylphosphine (DIP, 97%), quartz substrates and ammonium iodide (NH4I, ≥99%) were purchased from Sigma-Aldrich. Oleylamine (OLA, 8020 ACS Paragon Plus Environment

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90%), ammonium iodide (NH4I, 99%), acetonitrile (99.9%) and N,N-dimethylformamide (DMF, 99+%) were purchased from Acros Organics. Chloroform (HPLC grade), toluene (HPLC grade), hexane (HPLC grade), indium tin oxide (ITO) coated glass slides, and plain glass slides were purchased from Fisher Chemical. All chemicals were used without further purification. All reactions were performed under inert conditions using standard glovebox and Schlenk-line techniques. Preparation of precursor solutions. 0.5 M cadmium oleate [Cd(OA)2] was prepared by reacting 10 mmol of CdO with 10 mL of OA and 10 mL of ODE at 280 °C until the solution became colorless. 2 M n-trioctylphosphine selenium (TOPSe) and 2 M n-trioctylphosphine sulfur (TOPS) solution were prepared by mixing 10 mmol of selenium shot or 10 mmol sulfur powder, respectively, with 5 mL of TOP at room temperature overnight. Synthesis of CdSe QDs. 0.8 mL of 2M Cd(OA)2 and 20 mL of ODE were placed into a 100 mL 3-neck round-bottom flask. After degassing at 120 °C for 10 min and backfilling with N2, the temperature was elevated to 310 °C. When temperature reached 300 °C, 0.4 mL of 2M TOPSe was rapidly injected. Following that, 4 mL of TOP was added dropwise after 30 seconds to prevent precipitation of QDs. After 4 minutes, a mixture of 8 mL of 0.5 M Cd(OA)2, 2 mL of 2 M TOPSe, and 6 mL of ODE was continuously added to the reactor with a 20 mL/hr injection rate. At the end of the injection the reactor was rapidly quenched to room temperature. The asprepared solution was further purified by precipitation by addition of acetone, and redispersion in toluene. At the end, the precipitate was dried under N2 atmosphere, and dispersed in anhydrous hexane (concentration: 10 mg mL-1). Synthesis of CdSe/CdS QDs. A quarter part of the CdSe QDs dispersed in hexane was mixed with 10 mL of TOA and degassed at 120 °C for 10 min to remove hexane, and any oxygen


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and water. After backfilling with N2, 2 mL of 2 M Cd(OA)2 was added and temperature was increased to 300 °C. At 280 °C, 0.6 mmol of DDT was slowly added for 10 min, and the temperature was maintained for 60 min to grow the CdS shell. Precursors were added as single injections at 60 minute intervals according to the following schedule: 2 mL of Cd(OA)2 and 1 mmol of DDT; 3 mL of Cd(OA)2 and 1.5 mmol of DDT; 4 mL of Cd(OA)2 and 2 mmol of DDT; 5 mL of Cd(OA)2 and 2.5 mmol of DDT. At the end of the reaction, the reactor was quenched to room temperature and purified four times via precipataion/redispersion using ethanol and hexane. Finally, the precipitate was dried under air and dispersed in octane (concentration: 40 mg mL-1). Synthesis of ZnS QDs. 2 mmol of Zn(ac)2, 2 mL of OA, and 3 mL of ODE were reacted at 120 °C for 30 min under vacuum to prepare zinc oleate. After the temperature was increased to 300 °C, 1.5 mL of 2 M TOPS was swiftly injected and reacted for 10 min. After terminating the reaction, the as-prepared solution was purified three times via precipitation/redispersion using ethanol and toluene. The final precipitate was dried under air and re-dispersed in octane (concentration: 10 mg mL-1). Synthesis of PbSe QDs. In a 100 mL three-neck flask, 8 mmol PbBr2, 8 mL OLA, and 16 mL ODE were mixed and degassed at 120 °C for 20 minutes until optically clear. The temperature was raised to 180 °C and a mixture solution of 2mL OLA, 1 mL 2M TOPSe, and 0.1 mL DIP solution was injected into the flask. The reaction was quenched by removing the heating mantle after injection. After cooling to room temperature, PbSe QDs were precipitated by adding 30 mL of chloroform and 10 mL of acetonitrile. 10 mL of hexane was added to the precipitate to redisperse the QDs. The slurry solution containing unreacted lead precursor was centrifuged, to leave PbSe QDs in the supernatant. PbSe QDs were subsequently washed one more time via


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precipitating by adding 20 mL chloroform and 10 mL acetonitrile, centrifuging and finally dissolving in hexane for ligand exchange. Ligand exchange of PbSe QDs with NH4I. In a 50 mL centrifuge tube, NH4I powder was dissolved in 10 mL DMF to obtain solution with concentration of 50 mg/mL, which was mixed with PbSe solution (10 mL, 50 mg/mL) to form a double-layer. After shaking vigorously for 5 seconds, PbSe QDs were transferred from the top nonpolar phase to the bottom polar phase, and then were precipitated out by adding 10 mL toluene. The precipitated PbSe QDs were washed one more time by dissolving 10 mL of DMF and 10 mL of chloroform and finally dissolved in DFP solution with a concentration of ~100 mg/mL. Thin film fabrication. A quartz substrate was soaked into isopropanol in an ultrasonic bath for 30 minutes. For Au-coated substrates, 10 nm of Cr (for improved adhesion) and 200 nm of Au layers were thermally evaporated under a 10-7 Torr vacuum after the substrate was plasmacleaned for 15 minutes. QD films were fabricated via layer-by-layer deposition using NH4I as a surface ligand; after spin-coating the QD dispersion in octane at 2000 rpm for 30 sec, the film was fully covered with 50 mM NH4I in methanol for 1 min. Subsequently, the remaining NH4I solution was removed by spinning at 2000 rpm and washed using methanol three times to remove excess NH4I in the film. This QD film formation/surface treatment/washing procedure was repeated 10 times to increase film thickness up to ~1 micron. At the end of fabrication, the film was dried at 90 °C for 10 min to remove remnant solvent. All fabrication was conducted under inert atmosphere to prevent degradation of QDs. Photoemission experimental setup. To characterize photoemission from these films, the custom electron gun depicted in Figure 1 was used. The shaped electrode, biased at -20kV, provided electrostatic focus of the emitted electron beam enough to allow collection 20 cm away


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at a Faraday cup; current was measured using a low-noise picoammeter (Keithley 6517B electrometer). The system was optimized to allow the entire cycle of loading, evacuation and optical-power-dependent photocurrent studies of a single film to be performed within a period of several hours. A 1 kHz repetition rate amplified Ti:Sapphire laser system (Spitfire, SpectraPhysics) with either fundamental, second or third harmonic output was used to excite photoemission. The excitation was slightly focused on the QD film to the diameter of ~1.5 – 2 mm. Optical filters were used for control over the excitation power. Field effect transistor (FET) measurements. p++−silicon substrates with 300 nm SiO2 were cleaned by sonication in isopropanol for 30 min, and dried at 120 ºC for 10 min. 50 nm-thick CdSe, CdSe/CdS, and ZnS QD films were deposited on top of the substrate with same experimental procedures described above. At the end of fabrication, samples were dried at 120 ºC under N2 atmosphere for 30 min to remove residual solvent. Al source and drain electrodes separated by a 100 µm channel were thermally evaporated on top of QD films under 10-7 Torr vacuum. Electrical properties were measured using an Agilent B1500A semiconductor analyzer. For comparison, mobilities were also obtained under illumination using an ultraviolet lamp (254 nm): no substantial departures from dark mobilities were observed. Supporting Information Additional figures on the repeatability of QD cathode properties and the effect of cathode substrate on performance; calculations on resonant and nonresonant 4-photon absorption processes. Acknowledgments This work was supported by Los Alamos National Laboratory’s Laboratory Directed Research and Development (LDRD) program. 24 ACS Paragon Plus Environment

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