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Engineering Efficient Photon Upconversion in Semiconductor Heterostructures Christopher C. Milleville, Eric Y. Chen, Kyle R. Lennon, Jill M. Cleveland, Abinash Kumar, Jing Zhang, James A. Bork, Ansel Tessier, James M. LeBeau, D. Bruce Chase, Joshua M. O. Zide, and Matthew F. Doty ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07062 • Publication Date (Web): 21 Dec 2018 Downloaded from http://pubs.acs.org on December 21, 2018

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Engineering Efficient Photon Upconversion in Semiconductor Heterostructures Christopher C. Milleville,†,‡ Eric Y. Chen,†,‡ Kyle R. Lennon,¶ Jill M. Cleveland,† Abinash Kumar,§ Jing Zhang,† James A. Bork,† Ansel Tessier,k James M. LeBeau,§ D. Bruce Chase,† Joshua M. O. Zide,† and Matthew F. Doty∗,† †Department of Materials Science and Engineering, University of Delaware, Newark, DE 19716 USA ‡These two authors contributed equally to this work and are co-first authors. ¶Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE 19716 USA §Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27606 USA

kThe Tatnall School, Wilmington, DE 19807, USA E-mail: [email protected]

Abstract Photon upconversion is a photophysical process in which two low-energy photons are converted into one high-energy photon. Photon upconversion has broad appeal for a range of applications from biomedical imaging and targeted drug-release to solar energy harvesting. Current upconversion nanosystems, including lanthanide-doped nanocrystals and triplet-triplet annihilation (TTA) molecules, have achieved upconversion quan-

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tum yields of order 10-30%. However, the performance of these materials is hampered by inherently narrow absorption cross-sections and fixed energy levels originating in atomic, ionic, or molecular states. Semiconductors, on the other hand, have inherently wide absorption cross-sections. Moreover, recent advances enable the synthesis of colloidal semiconductor nanoparticles with complex heterostructures that can control band alignments and tune optical properties. We synthesize and characterize a three-component heterostructure that successfully upconverts photons under continuous-wave (CW) illumination and solar-relevant photon fluxes. The heterostructure is composed of two cadmium selenide (CdSe) quantum dots (QDs), an absorber and emitter, spatially separated by a cadmium sulfide (CdS) nanorod (NR). We demonstrate that the principles of semiconductor heterostructure engineering can be applied to engineer improved upconversion efficiency. We first eliminate electron trap-states near the surface of the absorbing QD and then tailor the bandgap of the NR such that charge carriers are funneled to the emitting QD. When combined, these two changes result in a 100-fold improvement in photon upconversion performance.

Keywords: solar energy, upconversion, nanostructures, semiconductors, core/rod/emitter, coupled quantum dots

The development and realization of nanoscale heterostructures that upconvert photons efficiently has broad appeal. 1–5 In biomedical applications, for example, an upconverting nanostructure could absorb deep-penetrating near-infrared photons and emit highenergy photons that trigger photodegradation of molecules or polymers for targeted-drug delivery. 6–8 For photovoltaic (PV) applications, photon upconversion could be utilized to improve solar energy conversion efficiency beyond the Shockley-Queisser limit by harvesting low-energy photons that are wasted in a traditional single-junction PV device and converting them to high-energy photons that can be absorbed by the host cell, as

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schematically depicted in Fig. 1a. 9–11 The ideal upconverting nanostructure for biomedical applications should have a narrow absorption band in the range of 700-900 nm 12 and would efficiently emit in the range of 300-450 nm. The ideal upconversion structure for solar energy harvesting applications, on the other hand, should have a spectrallybroad absorption cross-section, and absorption and emission energies that could be tuned to match the solar spectrum and commercially-available PV or solar fuel generation devices. 13 Consequently, there is great interest in the development of a flexible and tunable material platform for efficient photon upconversion. Lanthanide-doped nanocrystals have been studied extensively for optically triggered drug delivery and bioimaging 3 and have been incorporated into PV devices to improve solar energy conversion efficiency. 14 Similarly, photochemical upconverters such as TTA molecules have been extensively explored for solar energy harvesting applications. 15 Although these materials are well-established and have demonstrated relatively high quantum efficiency, 1 in both cases the upconversion mechanism relies on the absorption of discrete atomic, ionic, or molecular transitions. 15–17 Moreover, both of these materials harvest two photons of nearly the same energy in the upconversion process. These properties create major limitations on the spectral bandwidth of absorption and the ability to engineer absorption and emission wavelengths for different applications. Semiconductors, on the other hand, have inherently broad absorption at energies above the bandgap. Moreover, the size- and composition-dependent optical properties of semiconductor nanocrystals present an opportunity to engineer both quantum efficiency and absorption/emission wavelengths to meet the performance demands of photon upconversion applications. 18–21 Indeed, semiconductor colloidal QDs were recently used as a sensitizer for TTA molecules, allowing the system to reach an upconversion quantum yield of nearly 10%. 22 Here, we define the internal upconversion quantum efficiency (iUQE) as the ratio of the number of upconverted photons to the number of pairs of absorbed photons (a maximum of 100%), precisely double that of the upconversion quantum yield. Nanoscale

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Figure 1: Approach for bandgap engineering and application of upconverter for photovoltaics. (a) Schematic representation of a solar cell backed by an upconverter. (b) Four band diagrams representing our initial ("Sample A"), optimized ("Sample D") and two intermediary ("Sample B" and "Sample C") heterostructures. Highlighted in yellow is the ratio of the internal upconverter quantum efficiency to the total PL QY from the emitter QD. (c) Two digital pictures of upconversion under 750 nm, 2 W/cm2 (60-suns-effective*) and 0.03 W/cm2 (1-sun-effective*) CW excitation, both taken with a 650nm and 700nm shortpass filter, 1/4s and 30s shutter speed, respectively. *The term ‘effective’ denotes excitation with a number of photons equivalent to the number of photons in the AM1.5G solar spectrum between 600 and 860nm. (d) Corresponding representative HAADF STEM image (scale bar: 10nm) of the optimized heterostructure. (e) Schematic of absorbed lowenergy photons and emitted high-energy photons overlaid on a core/rod/emitter diagram semiconductor heterostructures that directly absorb and upconvert low-energy photons have been proposed, modeled, and realized. 13,23–25 A one-photon upconversion process (i.e. thermally-assisted upconversion) has been previously observed in single QDs when the photon absorption is followed by absorption of thermal energy to promote the carrier to higher-energy surface or bulk states, followed by radiative recombination. 26–28 Here, we focus on two-photon upconversion processes for improving solar cell performance. For example, we used a detailed balance model to estimate that net solar energy conversion efficiencies in excess of 40% are possible for wide-bandgap solar cells backed with 4


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an upconversion nanostructure based on indium arsenide (InAs) QDs and a graded InAlBiAs layer that suppresses both radiative and nonradiative loss pathways. 13,23 Oron and coworkers have reported colloidal semiconductor heterostructures that successfully upconvert, 24,25 but the upconversion mechanism in these colloidal heterostructures is very inefficient, requiring high intensity, pulsed excitation. Recently, Makarov et al. reported photon upconversion in thick-shell PbSe/CdSe QDs under low-intensity, continuouswave (CW) excitation. 29 However, upconversion in Makarov’s materials occurred via Auger recombination, utilizing two photons of the same energy, which severely limits the range of the solar spectrum that could be absorbed for energy harvesting applications. In this work, we apply the principles of semiconductor heterostructure engineering to systematically improve the upconversion performance of colloidal upconversion heterostructures, resulting in a 100-fold improvement in upconversion performance (see Fig. 1(b)). We demonstrate photon upconversion in a colloidal semiconductor heterostructure utilizing photons of different energy under CW excitation and at solar-relevant photon fluxes. The results illustrate a promising path toward the realization of a semiconductor paradigm for efficient photon upconversion.

Results/Discussion The upconversion heterostructure we are synthesizing is a three-component system, consisting of two QDs with different bandgaps spatially separated by a wider bandgap NR, as shown schematically in Fig. 1b. The smaller bandgap QD, composed of CdSe(Te), serves as an absorber of low-energy photons while the larger bandgap QD (CdSe) serves as the emitter. The upconversion process is based on the formation of a quasi-type-I/typeII band alignment resulting in a double well potential in the valence band and a continuous, nearly flat, conduction band. Conceptually, the upconversion process proceeds as follows, with reference to the top diagram of Fig. 1b. Absorption of a low-energy photon

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in the smaller bandgap QD generates an electron-hole pair (1). The electron delocalizes over the entire nanoparticle or escapes to the emitter QD (2), while the hole is confined to the valence band. Absorption of another photon through an intraband excitation (3) allows the hole to escape to the larger bandgap QD (4), resulting in radiative recombination with emission of a high-energy photon (5). To synthesize our initial heterostructure, shown in the top panel of Fig. 1b, we first synthesize tellurium-doped cadmium selenide QDs (CdSe(Te) with approximately 10 % Te and a diameter of ∼5 nm). These QDs are then used as seeds for the growth of cadmium sulfide (CdS) NRs (∼30 nm in length). We note that this CdS rod grows at an unequal rate from two opposing crystal facets of the CdSe(Te) seed, as described in detail in the supporting information. This results in the CdSe(Te) seed being located approximately 1/3 of the way along the resulting rod. A successive ionic layer adsorption and reaction (SILAR) process is then used to deposit small amounts of Cd and Se on the more reactive (longer) tip of the NR to form a CdSe QD. Photoluminescence (PL) spectroscopy of these heterostructures under 405nm CW light, shown by the black line in Fig. 2d, demonstrates that there are two peaks: one peak centered at approximately 800 nm and associated with recombination between the electron and hole in the CdSe(Te) absorber core and a second peak centered on approximately 575 nm and associated with recombination between the electron and hole in the CdSe emitter QD. A systematic study of the absorption and emission spectra as the heterostructure is grown is presented in the supplemental material. A typical scanning transmission electron microscopy (STEM) image of the heterostructure is shown in Fig. 1d. We schematically represent the CdSe(Te) QD in Fig. 1b with a quasi-type-I interface because the initial CdSe(Te) QD synthesis procedure 24 results in a composition gradient with a Te-rich center. This composition gradient arises from differences in the reactivity of Se and Te towards Cd: CdTe grows approximately twice as fast as CdSe. 30 Under the stoichiometric conditions of our initial CdSe(Te) synthesis, the nucleated QD is rich in Te due to the faster growth rate. As the Te precursor is depleted, CdSe growth dominates,

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Figure 2: Mapping and Optical Characterization of Optimized Heterostructure. (a−i) HAADF STEM images (scale bar: 5nm) and corresponding EDS maps of Sample A (a, c) and Sample D (b, d). A single nanorod (Sample D), with STEM-EDS maps of Cd, S, Se, Te, showing mixed Se/S in the rod region of the full QD-rod-QD structure (e−i). Ensemble absorption spectrum (red line) compared to upconversion PL intensity (black squares) as a function of excitation wavelength (735−850nm) (j). Ensemble PL spectrum overlaid with upconversion PL spectrum for optimized heterostructure (Sample D) (k). Insets are digital pictures (ISO-1600, f/3.5 (1.8), 1 second (30 seconds) shutter speed) showing visibly observable upconversion under 2 W/cm2 (60-suns-effective, 0.25s integration time, light green border) and 0.03 W/cm2 (1-sun-effective, 60s integration time, dashed green border) irradiation through a quartz vial viewed from the side through 650nm and 700nm shortpass filters.

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resulting in the composition gradient. This composition gradient is detrimental to upconversion performance because CdSe has a lower conduction band than CdSe (Te), resulting in lower energy states near the surface that can trap electrons and prevent transfer across the NR to the CdSe QD emitter. In our first approach to improve upconversion performance we developed a procedure to synthesize CdSe(Te) QDs with a homogeneous composition under cadmium-limited conditions by using a 5x excess of Se/Te. Under cadmium-limited conditions, CdSe and CdTe growth rates remain equal because all Cd is consumed well before the Se or Te is depleted, resulting in a homogeneous composition. We test the efficacy of this improvement by completing the synthesis of the remainder of the heterostructure using the same methods discussed above. Our second approach to engineering improved upconversion performance was to increase the probability of radiative recombination within the CdSe emitter QD by creating a tapered bandgap within the CdS NR spacer. This tapered bandgap "funnels" charge carriers away from the CdSe(Te) absorber and toward the CdSe QD emitter. To realize this tapered bandgap we developed a method to synthesize CdS1-x Sex NRs with increasing Se percent composition along the length of the NR. Details on all of our synthesis procedures can be found in the supporting information. Energy-dispersive X-ray spectroscopy (EDS) was used to characterize the CdS1-x Sex , as shown in Fig. 2d. In the non-alloyed CdS rod in Fig. 2c, S is isolated to the rod, while Se is concentrated in the absorber/emitter. In the alloyed rod, shown in Fig. 2d, both Se and S are present along the length of the rod. We note that our synthesis method intentionally increases the Se fraction as the rod grows and a detailed STEM-EDS map of a single nanorod indicates that Se is present throughout the nanorod. However, the STEM-EDS maps cannot resolve the existence of a gradient towards either end of the rod. We expect that the ensemble of particles has an inhomogeneous variation in Se/S locations and concentrations, leading to an inhomogeneous variation in the magnitude of the bandgap decrease and the resulting funneling effect. A complete spread of STEM-EDS maps and line scans for Samples A and D can be found in

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the supporting information section. We characterize the photon upconversion performance of these heterostructures using solutions of nanoparticles dispersed in toluene and excited with CW light between 735 and 850 nm at excitation intensities up to 30 W/cm2 . This range of excitation wavelengths is chosen such that only the CdSe(Te) absorber QD has a sufficiently small bandgap to absorb the incident photons. We measure photoluminescence intensity emitted in the range from 450−650 nm. We take great care to spectrally filter the excitation source to ensure that no higher energy photons are present, thus ensuring that any observed emission in the range 450−650 nm must be generated by the upconversion process. The spectrum of the upconverted photoluminescence (UCPL) for the optimized heterostructure, plotted in light green in Fig. 2k, clearly matches the ensemble PL peak associated with the CdSe emitter. To assess the spectral width of the absorption we measure the UCPL intensity as a function of excitation photon energy, as shown in Fig. 2j. The UCPL intensity is proportional to the ensemble absorption spectrum (see supporting information), further evidence that direct excitation of the CdSe(Te) QD leads to the UCPL from the CdSe QD. We have verified that the upconversion structures harvest both inter-band (process 1 in Fig. 1(b)) and intra-band (process 3) low-energy photons by measuring the UCPL under two-color excitation conditions as described below and in the supporting information. To verify that the upconversion process involves the absorption of two low-energy photons, we compare the UCPL intensity of the initial (i.e. control structure) and optimized heterostructure (i.e. best performing heterostructure from Fig. 1(b)) under CW 750 nm excitation as a function of the excitation fluence. The results are plotted on a log-log scale in Fig. 3a for the initial and optimized heterostructures, respectively. The super-linear dependence of UCPL intensity on excitation fluence, at low fluences, provides clear evidence that more than one photon is involved in the upconversion process. In this case, the 750 nm photons are driving both the intra- and inter-band absorption processes. We observe a slope between 1 and 2 at low excitation fluences, suggesting a combination of

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one-photon and multi-photon upconversion. We return to this point below. At higher excitation fluences we observe a linear power dependence (slope = 1). This linear slope indicates saturation of the interband absorption process due to the finite density of states in the absorber QD. 24 We determine that saturation occurs at 3 W/cm2 for both initial and optimized heterostructures. We then repeat the upconversion measurements using two CW excitation wavelengths, 810 nm and 980 nm. The results are shown in Fig. 3c and d. 980 nm photons do not have sufficient energy to generate electron-hole pairs in the absorber QD, and thus no UCPL is observed when no 810 nm excitation is present. For a fixed 50 W/cm2 fluence of 810 nm excitation, we observe that the UCPL intensity increases nearly linearly with the excitation fluence of the 980 excitation. This verifies that the upconversion process harvests two different bands of the solar spectrum, with the 980 nm photons driving the intra-band excitation process. Finally, we repeat the UCPL measurements with a CW 750 nm excitation intensity equivalent to the photon flux in the unconcentrated AM1.5G solar spectrum in the wavelength range between 600 and 860nm, which is the range that could be harvested by the upconverters discussed here. This wavelength range contains approximately 30% of the total number of photons in the AM1.5G spectrum, which we define as ‘1-sun-effective.’ We therefore use a monochromatic excitation fluence of 0.03 W/cm2 instead of the standard 0.1 W/cm2 . 31 The results, shown by the open green circles in Fig. 2k, verify that the upconversion process remains viable at photon fluxes relevant to solar energy harvesting devices. Direct measurement of upconversion under an AM1.5G solar simulator has not yet been possible due to the challenges of filtering out all high energy photons to the level required to guarantee that the observed results can only be attributed to upconversion. We now turn to analyzing the improvement in upconversion performance that results from our application of semiconductor heterostructure engineering techniques as schematically depicted in Fig. 1b. In Fig. 3b we plot the PL (dashed, under 1.4 W/cm2 405nm CW excitation) and UCPL (solid, under 2.8 W/cm2 750nm CW excitation) for the

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initial (red) and optimized (blue) heterostructures. We observe three important changes. First, the initial structure displays a very broad UCPL peak, which we believe indicates carriers are not well localized in the CdSe emitter QD before recombination. Second, we observe a significant increase in UCPL intensity relative to PL intensity in the optimized structure. We will return to this point below. Third, we observe a decrease in the ensemble PL intensity for the optimized heterostructure. We believe this decreased PL, for the optimized heterostructure, originates in an increase in nonradiative recombination sites on the surface of the heterostructure, which are known to be one of the major limitations to PL emission efficiency in conventional colloidal heterostructures. 32,33 To quantify the improvement in upconversion performance that results from our application of semiconductor heterostructure engineering methods, we measure the quantum yield of both the PL and UCPL emission under 1 W/cm2 405 nm and 20 W/cm2 750 nm CW excitation, respectively. The excitation fluence at which the PLQY is measured does not saturate the sample (data not shown here), but the iUQE is measured beyond the saturation point in both structures A and D (see Fig. 3a). We do expect higher iUQE to be measured with increasing power, based on experimentation and modeling (see supporting information). We do not expect degradation of the material due to the absence of bleaching and expected diffusion of heat away from nanoparticles in the presence of toluene. From these measurements we compute the iUQE of the upconversion heterostructures. Details of this calculation can be found in the supporting information and the results are summarized in Table 1. The iUQE of the initial heterostructures is 0.0002 % and improves to 0.002 % for the optimized heterostructure. While this demonstrates a factor of 10 improvement in absolute quantum yield, the iUQE is limited by two factors. The first is the efficiency of the upconversion process itself. The second is nonradiative loss at the emitter QD, which affects both PL and UCPL. As noted above, we observe significant changes in the PL QY from sample to sample, changes that likely originate in variable concentrations of surface defects because these upconversion structures

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Figure 3: Linear and quadratic power-dependence under single- and two-color excitation. (a) Power-dependence plots and corresponding linear fits before (green line) and after (red line) the saturation limit for Samples A (red squares) and D (blue diamonds). (b) Ensemble PL spectra (dotted lines) compared to upconversion PL spectra (solid lines) for Samples A (red) and D (blue). (c) Improved upconversion photoluminescence intensity via two-color excitation for the optimized sample, with UCPL spectra for 980nm only (0 W/cm2 810nm CW excitation) overlaid with 810nm only (0 W/cm2 980nm CW excitation) and with increasing 980nm CW fluence (30,...,177 W/cm2 ). (d) Percent increase in integrated UCPL intensity (of FWHM) versus 980nm CW excitation fluence (W/cm2 ), with error bars calculated by integrating the change in UCPL intensity under 810nm only over time. Log-log fit of 0.78 is given with 95% confidence interval.

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are capped with passivating ligands and not crystalline shells. To isolate and quantify changes in the efficiency of the upconversion process itself, we take the ratio of the iUQE to the PL QY of the CdSe emitter, again reported in Table 1 and summarized in Fig. 4b. This ratio normalizes the iUQE by the nonradiative losses that are present in each sample. We find that the iUQE / PL QY ratio improves from 0.05% to 6% as we progress from the initial to optimized heterostructures. This demonstrates more than 100x improvement in the efficiency of the upconversion process. The photos in Fig. 4b illustrate this improvement visually. The top insets to Fig. 4b are real-color photos of the UCPL emitted from all full-structure samples through a 650nm and 700nm SP filter under CW excitation by 750nm laser with photon flux of 0.1 W/cm2 (Samples A - C) and 0.03 W/cm2 (Sample D), equivalent to 3-/1-sun-effective. Yellow UCPL is visible to the naked eye only in the optimized heterostructure and is spectrally distinct from the red, one-photon upconversion PL present in all other samples, a point we return to below. Finally, we analyze the factors that may be limiting the upconversion efficiency of these structures in order to identify potential routes to engineering further improvements. The first factor we suspect is limiting performance is nonradiative recombination losses associated with defects. Although we cannot perform a simple control experiment to verify the impact of surface defects, the importance of crystalline, inorganic passivation for high PL QY is clearly established in the literature. 33 For example, organic ligand-passivated CdTe QDs have a modest QY of 24% that improves to as much as 92% when a CdS inorganic shell is applied. 34 Several groups have studied core-multishell CdSe/CdS/ZnS QDs and reported improvement in both photoluminescence QY and crystal quality. 35,36 However, adapting these procedures to rod-like structures has proven more difficult and can alter the electronic properties of the underlying structure. 37–39 Efforts to develop a synthetic procedure to apply a wide bandgap passivating shell without damaging our underlying structure are underway. The second factor we consider is the effects of phonons. As reported in Fig. 3a, we

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observe that the UCPL intensity as a function of excitation laser fluence has a slope between 1 and 2 at low excitation fluences, suggesting a combination of one-photon and two-photon upconversion processes. To explore the contributions of phonons, we conducted low-temperature PL and UCPL measurements. Details of the experimental methods can be found in the supporting information. Fig. 4c shows the normalized PL of the optimized structure as a function of temperature. The first observed effect of decreasing temperature is the typical blue-shift of the PL emission peaks as the material bandgap increases. The second observed effect of decreasing temperature is the emergence of a second PL peak centered around 610nm. Lower energy PL peaks such as this are often associated with shallow defects or trap states. 32 At higher temperatures, such states are not always evident because thermal excitation depopulates the states more rapidly than radiative recombination. 40 The third, and perhaps most important, observed effect of decreasing temperature is that the PL from the emitter QD (550−650 nm, see inset of Fig. 4c) becomes significantly more intense relative to the PL peak associated with the absorber QD (800 nm). The fraction of excited carriers that relax into the emitter rather than the absorber QD should be independent of temperature. The increase in the intensity of the emitter PL relative to the absorber PL therefore suggests that low temperatures inhibit the escape of carriers from the CdSe emitter to the CdSe(Te) absorber. This conclusion is supported by preliminary rate equation modelling of the iUQE of this structure, as described in the supplementary material. These models predict that the iUQE of the structure will show significant improvement when the temperature is reduced from 300K to 80K. By systematically turning on and off mobility and diffusion parameters in the model, we confirm that the predicted improvement at lower temperature originates in the suppression of thermally-driven diffusion of holes from the emitter QD to the absorber QD (see Fig 4b, which plots the normalized emitter PL intensity). This dependence is qualitatively consistent with the predictions of the model. We note that comparisons to PLQY and iUQE are not made at low temperature due to experimental limitations when the

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colloidal solution is drop-cast onto a silicon wafer. For example, sample bleaching under constant laser illumination occurs within several seconds due to the aggregation of un-passivated nanoparticles. 41 The aggregation may also result in lower signal due to a greater number of non-radiative recombination pathways via charge transfer between nanoparticles. Despite this limitation, the comparison of temperature-dependent PL data to the model calculations provides an important signal that thermal escape of carriers from the emitter QD is a loss pathway that must be managed. The model results suggest that a steeper gradient for the funnel region of the nanorod is one change that could suppress this thermal escape.

Conclusions In summary, we have demonstrated that colloidal semiconductor heterostructures utilizing absorber and emitter QDs separated by a nanorod spacer can implement photon upconversion under CW illumination and photon fluxes equivalent to the unconcentrated solar spectrum. Furthermore, we have demonstrated that semiconductor heterostructure engineering principles typically employed within epitaxially-grown material platforms can be adapted to colloidal synthesis. We demonstrate that the systematic employment of these methods achieves a 100-fold improvement in the upconversion performance of these heterostructures. Finally, we analyze the present performance of these materials and identify several factors that are presently limiting performance, including surface defects and thermally-driven diffusion of carriers from the emitter to the absorber QD. Improvements by a factor of as much as 100 can be expected when surface defects are passivated by wide bandgap shells. 34 Our understanding of the carrier transfer dynamics and their impact on upconversion efficiency will guide the continued application of semiconductor heterostructure engineering methods to improve the upconversion performance. Moreover, our improved understanding of the bandgaps and band alignments required for

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Figure 4: Quantifying upconverter performance and limitations. (a) Photoluminescence quantum yield (PLQY) and internal upconversion quantum efficiency (iUQE) (zoomedin x500 inset) of Samples A (red) and D (blue), relative to the rhodamine101 (orange) standard PLQY (zoomed-out x0.2 inset). (b) Steady-state PLQY (open black squares), iUQE (black square) and corresponding "iUQE/PLQY" ratio (blue squares), for all four heterostructures. Lines are intended to guide the eye. Top inset: photos of visible UCPL (under 0.1 W/cm2 750nm CW excitation, ISO-2000, f/5.6, 30s, with a 650nm and 700nm SP) from Samples A-D (left-right). (c) Temperature-dependent steady-state photoluminescence spectra of Sample D, normalized to the absorber QD PL, with inset showing CdSe QD emitter PL. (d) PL intensity vs. 1/kB T (colored squares), fit with an exponential including 95% confidence, Inset: High temperature (100K - 298K) data points expanded for clarity.

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efficient photon upconversion now make it possible to tailor the absorption and emission wavelengths for specific applications while retaining reasonable upconversion efficiency.

Methods/Experimental Nanoparticle Synthesis. Te-doped CdSe QDs were synthesized by adaptation of a previously reported method. 24 Composition of the CdSe(Te) QDs was controlled by varying the Se/Te:Cd molar ratio. CdSe(Te) QDs with a gradient and a homogenous composition were synthesized under stoichiometric and Cd-limited conditions, respectively. CdS and CdS1-x Sex NRs were grown over CdSe(Te) QDs using a seeded growth approach, using a procedure modified from a previous report. 42 Se was incorporated into the CdS NRs by injection of small volumes of Se dissolved in trioctylphosphine (TOP) during the NR growth reaction. Successive ionic layer adsorption and reaction (SILAR) was used to grow a CdSe QD at the tip of the NRs. 24 CdSe QD growth was monitored using photoluminescence spectroscopy. Once the desired PL peak was obtained, the reaction was stopped, NRs were purified by solvent-nonsolvent washing, and then redispersed in toluene for optical characterization. Further synthesis details can be found in the supplementary information.

Electron Microscopy. Dilute dispersions of nanoparticles were drop-casted onto STEM grids (Cu/200 mesh/UL carbon film, Electron Microscopy Sciences). High angle annular dark-field scanning transmission electron microscopy and accompanying energy dispersive X-ray spectroscopy were performed on an FEI Titan G2 60-300 S/TEM operated at 200 kV. The convergence semi-angle was 19.6 mrad and the collection inner semi-angle was 77 mrad. Each EDS map was formed after subtracting the background from each spectrum. A Gaussian blur

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with a standard deviation of 4 pixels was applied to to each EDS map to reduce noise.

Optical Characterization. UV-vis absorption spectra of nanoparticle dispersions were obtained with an Ocean Optics 4000-UV-Vis spectrometer. Steady-state (SS) and upconversion (UC) photoluminescence measurements were taken of nanoparticles stored in square (4.5cm x 1.25cm x 1.25cm) cuvettes that were diluted to OD = 0.1-0.5 at 700 nm (the onset of mid-energy photon absorption) for minimal re-absorption between nanoparticles. For SSPL, a 405nm, 30mW CW laser diode was focused using a 4x objective lens (Olympus Plan Achromat, RMS4X) to excite carriers above the bandgap of all transitions in the nanoparticle samples. The emitted light was collected by a 10x microscope objective (Olympus Plan Achromat, RMS10X) and focused using an achromatic doublet ( f = 100 mm) onto the 50µm entrance slit of a Princeton Instruments Acton SpectraPro 2500i Spectrometer. A grating at 150 grooves/mm directs the beam towards a liquid nitrogen-cooled charge coupled device camera (1340 x 100 pixels). For single-color UCPL experiments, samples were excited with a tunable, Ti:sapph CW laser with a maximum power of 600mW and wavelengths below the bandgap of the emitter QD (735nm−850nm). For two-color experiments, 810nm CW light from the Ti:sapph was combined with 980nm CW light from a 5W 980nm laser diode using an 850nm longpass dichroic mirror. A shutter (SH1, Thorlabs) was placed in front of the 980nm laser diode to produce single-shot measurements. Multiple longpass (LP) filters (750nm and 2x700nm) were used to prevent above bandgap excitation. The UCPL emission was filtered with several shortpass (SP) filters (650nm and 3x700nm) to eliminate incident laser signal. Under near-infrared (NIR) excitation, the measured upconversion photoluminescence (UCPL) is visible by eye. Low-power measurements were approximated to the region of the AM1.5G solar spectrum

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Quantum Yield Determination. Quantum Yield (QY) measurements were performed using an integrating sphere coupled to a spectrometer and silicon CCD. The sphere and spectrometer are coupled via a roundround fiber bundle and round-linear fiber bundle, which are connected by an in-line filter mount for post-excitation filtering. One end of the round-round fiber bundle is inserted into the output port on the integrating sphere, and the linear end of the round-linear bundle is placed at an input slit for the spectrometer where it can be adjusted by an xyzmicrometer stage and 360-degree rotation mount. The wavelength dependent relative collection efficiency of the sphere-fibers-spectrometer setup is measured by integrating the collected laser signal at known laser power using various laser wavelengths. Further details on the QY method can be found in the supporting information. Table 1: Summary of upconversion heterostructure performance. Heterostructure CdSe PL QY grad. core/flat rod (Sample A) 0.4% homog. core/flat rod (Sample B) 0.3% grad. core/tapered rod (Sample C) 0.3% homog. core/tapered rod (Sample D) 0.04%

iUQE iUQE/CdSe PL QY − 4 2 · 10 % 0.06% − 4 8 · 10 % 0.3% − 3 1 · 10 % 0.3% 2 · 10−3 % 6%

Acknowledgement This study was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (ECCS- 1542015). AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI). C.M. synthesized the heterostructures. E.C. and K.L. performed optical experiments. A.T. built sample holder for optical measurements. J.C., J.Z., and J.B. performed simulations. A.K. and J.L. performed the HAADF STEM and STEM-EDS experiments and 19


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analysis. M.D., J.Z., and B.C. conceived and supervised the project. The authors declare that they have no competing financial interests. Correspondence and requests for materials should be addressed to M.F.D. (email: [email protected]).

Supporting Information Available Absorbance, PL, UCPL, and EDS maps (and line-scans) of the original (Sample A) and optimized (Sample D) heterostructures, PL quantum yield and internal UQE methods, a normalization procedure of the temperature-dependent PL, and the kinetic rate model of the coupled QD system under low and room temperature (with and without concentration), are included in the Supporting Information.

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