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Plasmon Dynamics in Colloidal Cu2xSe Nanocrystals Francesco Scotognella,*,#,† Giuseppe Della Valle,#,† Ajay Ram Srimath Kandada,#,† Dirk Dorfs,‡ Margherita Zavelani-Rossi,† Matteo Conforti,§ Karol Miszta,‡ Alberto Comin,^ Kseniya Korobchevskaya,^ Guglielmo Lanzani,†,|| Liberato Manna,‡ and Francesco Tassone|| †
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Dipartimento di Fisica, Istituto di Fotonica e Nanotecnologie CNR, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy ‡ Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy § CNISM and Dipartimento di Ingegneria dell’Informazione, Universita di Brescia, Via Branze 38, 25123 Brescia, Italy CNST of IIT@POLIMI, Via Pascoli 70/3, 20133 Milano, Italy ^ Nanophysics Unit, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova GE, Italy
bS Supporting Information ABSTRACT: The optical response of metallic nanostructures after intense excitation with femtosecond-laser pulses has recently attracted increasing attention: such response is dominated by ultrafast electronphonon coupling and offers the possibility to achieve optical modulation with unprecedented terahertz bandwidth. In addition to noble metal nanoparticles, efforts have been made in recent years to synthesize heavily doped semiconductor nanocrystals so as to achieve a plasmonic behavior with spectrally tunable features. In this work, we studied the dynamics of the localized plasmon resonance exhibited by colloidal Cu2‑xSe nanocrystals of 13 nm in diameter and with x around 0.15, upon excitation by ultrafast laser pulses via pumpprobe experiments in the near-infrared, with ∼200 fs resolution time. The experimental results were interpreted according to the two-temperature model and revealed the existence of strong nonlinearities in the plasmonic absorption due to the much lower carrier density of Cu2xSe compared to noble metals, which led to ultrafast control of the probe signal with modulation depth exceeding 40% in transmission. KEYWORDS: Plasmon resonance, semiconductor nanocrystals, pumpprobe, copper selenide, two temperature model, optical switching
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heoretical and experimental research on the optical features exhibited by metallic nanosystems has paved the way to extensive applications in several fields, from surface-enhanced spectroscopies to biological and chemical nanosensing.1 Such developments are primarily due to the localized surface plasmon resonance (LSPR), which results in intense optical absorption and scattering as well as subwavelength localization of large electrical fields in the vicinity of the nano-object.210 The plasmonic response of these metallic nanostructures is strongly dependent on the type of metal of which they are made, on the dielectric function of the surrounding medium, and on the particle shape and size. Nanoparticles of several metals, such as Ag, Au, Cu, and Pt, exhibiting plasmonic response in the ultraviolet and in the visible regions of the spectrum have been successfully synthesized in the past decades,1115 and elongated metallic nanoparticles have been shown to have plasmonic response in the near-infrared, due to excitation of the longitudinal plasmon mode.1618 In addition to “traditional” metal nanoparticles, a consistent effort has been made in the last years in the synthesis of heavily r 2011 American Chemical Society
doped semiconductor nanocrystals so as to achieve metallic behavior and eventually a tunable plasmonic response. In particular, research has focused on nanocrystals of copper chalcogenide, mainly Cu2S and Cu2Se.1926 It is also established that in these materials Cu can exist in stoichiometric ratios considerably lower than 2:1 with respect to Se or S; i.e., there can be a large number of copper vacancies, which results in a large number of free carriers (holes) in the valence band, hence in a “self-doping” of the material. The influence of this heavy doping on the optical response in the infrared region (IR) of copper chalcogenides has been a subject of interest very recently, although the observation of IR response from these materials dates several decades back. Already in 1973, Gorbachev and Putilin observed a plasmon band in the reflectivity of p-type copper selenide and copper telluride thin films.27 Around 10 years ago, Yumashev et al. and Gurin et al. investigated oxidized Cu2Se nanoparticles embedded into Received: July 13, 2011 Revised: September 20, 2011 Published: September 22, 2011 4711
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Nano Letters glasses, and they noticed too an IR band, which however was attributed to transitions of electrons trapped in long-lived deep trap states in the glass matrix.28,29 In 2009, Zhao et al. prepared Cu2xS nanocrystals with different values of x and clearly demonstrated how this had a strong influence on their IR plasmonic response.19 Early this year, Luther et al.24 and Dorfs et al.26 reported detailed studies on the plasmonic properties of Cu2xS nanoparticles24 and Cu2xSe nanoparticles.26 Both groups demonstrated that the gradual oxidation of particles with an initial 2:1 Cu:S(Se) stoichiometry increased the degree of copper deficiency “x”, thus yielding Cu2xS nanoparticles. As a consequence of this oxidation, an IR band emerged and increased in intensity and blue-shifted in energy depending on the extent of copper deficiency. Dorfs et al.26 additionally showed the reversible tuning of the IR plasmonic response of Cu2xSe nanocrystals via their gradual oxidation, either under air or by stepwise addition of a Ce(IV) complex and reduction, upon exposure to a Cu(I) complex, until the Cu:Se stoichiometry could be restored to 2:1. In addition to these developments of materials, in the very last years the study of the dynamical response of plasmonic nanostructures has led to the emergence of the field of ultrafast active plasmonics.30 It has been recently reported that direct optical excitation of the metal by intense femtosecond laser pulses induces an ultrafast modulation of plasmonic signals, which uncovers the unprecedented possibility to achieve terahertz modulation bandwidth, i.e., a rate at least 5 orders of magnitude faster than existing technologies.30 A quantitative investigation of such dynamical features is therefore critical for the development of a new generation of ultrafast nanodevices. Pumpprobe spectroscopy, giving access to the electronic excitation and subsequent relaxation processes in the material, is to date the most suitable tool for the experimental study of the ultrafast dynamical features exhibited by plasmonic nanostructures. So far, several noble metal structures have been investigated, including spherical nanoparticles3133 and nanorods,34 and all these studies have found that the underlying physical phenomena are actually similar to those observed in bulk (i.e., thin film) metallic systems.35,36 When the pump pulse is in the infrared region of the spectrum, the initial Fermi distribution of electrons in the conduction band is strongly perturbed by pump absorption; the pump pulse creates energetic electrons that are not in thermal equilibrium and within a few hundred femtoseconds a new Fermi distribution is reached via strong electronelectron scattering, resulting in a thermalized electron gas with higher temperature than the lattice (hot electrons). Subsequently, within the following few picoseconds the electron gas cools down by releasing its excess energy to the lattice through electronphonon coupling. Ultimately, within hundreds of picoseconds, the nanoparticle releases its energy to the environment,17 with heat conduction to the surface of the nanoparticles provided by phononphonon coupling. While all these processes have been studied in noble metal nanoparticles, not much is known instead in similar processes in the recently developed nanoparticles of heavily self-doped copper chalcogenides. In this work, we studied the optical response of colloidal Cu2xSe nanocrystals to ultrafast laser pulses, recorded via pumpprobe experiments in the near-infrared with ∼200 fs resolution time, and specifically investigated the holephonon coupling dynamics taking place on the picosecond time scale. The experimental results were interpreted according to the two-temperature model already developed for the theoretical
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Figure 1. Experimental steady-state optical extiction spectrum of a solution of Cu1.85Se nanocrystals dissolved in toluene (solid black line) and theoretically computed extinction efficiency (dashed red line) from eqs 1 and 2 (see text for parameters). The arrows indicate the wavelength of the optical pulses in the pumpprobe experiments. The inset reports a transmission electron microscopy (TEM) image of the nanocrystals.
investigation of more conventional metallic systems.35 We found relevant differences in these Cu2xSe nanocrystals with respect to the more conventional nanoparticles of noble metals, due to the much lower carrier density in the present particles. This translates into much larger carrier temperatures and strong nonlinearities in the plasmonic absorption of Cu2xSe nanocrystals. The Cu2xSe nanocrystals considered in the present study were synthesized and then controllably oxidized upon exposure to air as described in detail by Dorfs et al.,26 until x = 0.15 was reached, so their final composition was Cu1.85Se. The optical response exhibited by these nanocrystals under continuous wave excitation with near-infrared light is plasmonic in nature and can be quantitatively interpreted according to the quasi-static approximation of the Mie theory.26 More precisely, the dipolar absorption and scattering cross sections of a Cu2xSe nanoparticle of radius R , 2πc/ω are respectively given by εðωÞ εm σ A ¼ 4πkR 3 Im ð1Þ εðωÞ þ 2εm 2 8 4 6 εðωÞ εm σ S ¼ πk R εðωÞ þ 2εm 3 where εm = nm2 is the dielectric constant of the environmental medium, k = nmω/c (being ω the optical frequency) and ε = ε1(ω) + iε2(ω) is the Cu2xSe dielectric function provided by the Drude model ε1 ¼ ε∞ ε2 ¼
ω2P ω2 þ Γ2
ð2Þ
ω2P Γ ωðω2 þ Γ2 Þ
with ε∞ the high frequency dielectric permittivity, ωP the plasma frequency of the free carriers of the system, and Γ the free carrier 4712
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Figure 2. Differential transmission signal from a solution of Cu1.85Se nanocrystals dissolved in toluene for short-time dynamics at (a) 900 nm and at (b) 1300 nm probe wavelength and (c) long-time dynamics at 900 nm. The experimental results (solid lines) are compared with numerical calculations (dotted lines) for the different incident pump fluences.
damping (i.e., the inverse of the carrier relaxation time). We remark that the background dielectric constant ε∞ is due to the presence of interband transitions at higher photon energy. Previous results by Dorfs et al.26 showed that, depending on the degree of Cu deficiency (i.e., “x” in Cu2xSe), different material dielectric functions ε are retrieved, resulting in a plasmon resonance (at frequency ω0 approximately given by ε1(ω0) = 2εm) that blueshifts with increasing x. For the present study we selected Cu1.85Se nanocrystals dissolved in toluene (refractive index nm = 1.497, εm = 2.24), which were characterized by a relatively narrow plasmonic resonance (about 0.56 eV), with an intense peak around 1050 nm, as shown in Figure 1, which reports the optical extinction spectrum of a solution of Cu1.85Se nanocrystals in toluene (with estimated particle concentration N = 13 1013 cm3) placed in a 0.5 mm thick quartz cuvette. Figure 1 also reports the total extinction cross section σE = σA + σS of Cu1.85Se nanoparticles computed from eqs 1 and 2 with Drude parameters ε∞ = 10, ωP = 6.76 1015 rad/s, and Γ = 6.64 1014 rad/s. The pump probe setup was based on a commercial Ti: sapphire amplified laser system delivering 150 fs pulses at 1 kHz repetition rate at a central wavelength of 800 nm. A fraction of the beam was used to pump a non-collinear parametric amplifier (NOPA) to generate pulses in the near-infrared.37,38 We tuned the NOPA to obtain pump pulses in correspondence with the plasmonic resonance of the sample (∼1050 nm, 1.19 eV with a bandwidth of 4045 nm). The probe pulses were
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produced by focusing the fundamental beam into a 2 mm thick sapphire plate in order to generate a stable white light supercontinuum. A long pass filter with cut-on wavelength at 820 nm was used to filter out the residual fundamental and the visible components of the probe pulses. The pump and probe beams were focused onto the sample with a spot size of 200 μm. The pumpprobe setup employed a computer-controlled optical multichannel analyzer and the measured signal is a map of the chirp-free differential transmission ΔT/T = (Ton Toff)/Toff as a function of the pumpprobe time delay for different probe wavelengths; Ton and Toff are the probe spectra transmitted by the excited and unperturbed samples, respectively. Figure 2 reports the temporal dynamics of the relative differential transmission (ΔT/T) probed at 900 nm (Figure 2a) and at 1300 nm (Figure 2b), obtained by exciting the nanocrystals with different pump fluences. At 900 nm, the experimental results revealed ΔT/T > 0 right after the absorption of the pump beam, with a maximum value of about 40% under the maximum pump fluence of 4.45 mJ/cm2, whereas at 1300 nm ΔT/T < 0 was observed, with maximum (negative) value of about 3% under a pump fluence of 1.87 mJ/cm2. In both cases, a monotonic and fast decrease of the signal was then observed within a few picoseconds (Figure 2a,b) followed by a much slower (nanosecond time scale) decay (see Figure 2c for the signal probed at 900 nm), leading to complete recovery of the initial condition (before pump arrival) within a few nanoseconds. It is worth noting that the maximum ΔT/T exhibited by Cu1.85Se nanocrystals exceeds by at least 1 order of magnitued the ΔT/T reported in metallic systems for comparable pump fluences (cf. refs 33 and 35). To tentatively describe the nonlinear optical features as reported in Figure 2, we applied the theoretical methods developed for metals, since we investigated the wavelength region dominated by the metallic behavior of the system. The pump probe dynamics of metallic systems can be ascribed to the variations attained by the metal dielectric function induced by pump absorption.35,36 As explained before, the picosecond dynamics is related to the cooling of the hot electron gas generated by absorption of the pump energy. Therefore, the time evolution of the system within the picosecond time scale can be modeled as a result of a heat transfer between a thermalized gas of carriers at temperature TC and the lattice at a lower temperature TL, via carrierphonon scattering process. The socalled two temperature model (TTM) quantitatively accounts for such transfer according to the coupled equations35 8 dTC > ¼ GðTC TL Þ þ PA ðtÞ < γTC dt ð3Þ dT > : CL L ¼ GðTC TL Þ GL ðTL T0 Þ dt where γTC is the heat capacity of the carrier gas, being γ the socalled carrier heat capacity constant, CL is the heat capacity of the lattice, G is the carrierlattice coupling factor, GL is the latticeenvironment coupling factor, T0 is the environmental temperature (which is assumed constant) and PA(t) is the pump power density absorbed in the volume of the metallic system. Once the temperature dynamics induced by the pump pulse is determined from eq 3, the optical response of the systems to the probe pulse can then be retrieved from the temperature dependence of the dielectric function of the material and from the electromagnetic boundary conditions due to the particular 4713
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Δε∞ is basically real. In metals, it is well-known that this modulation is proportional to the carrier excess energy,33,36 and thus it scales quadratically with the carrier temperature TC Δε∞ ¼ ηTc2
ð4Þ
with η being a fitting parameter. The subsequent heating of the lattice also results in a modulation of ε∞, which is however negligible in our case, and in a modulation of the ωp and Γ related to the free carriers. By measuring the steady-state optical extiction spectrum of the Cu1.85Se nanocrystals at different temperatures, we found that the variation of ωp with TL is negligible, whereas the broadening Γ increases linearly with TL as in metallic systems36 Γ ¼ Γ0 þ βðTL T0 Þ
Figure 3. (a) Tentative sketch of band diagram in Cu1.85Se for optical transitions in the visible. The smearing of the electron distribution f(E) due to intraband pump absorption and the subsequent carrier temperature (TC) increase is also illustrated. (b) Extinction cross section of Cu1.85Se nanospheres at different lattice temperatures (TL) resulting from the linear temperature dependence of the free carrier damping Γ (shown in the inset) as experimentally determined from temperature dependent spectroscopic measurements (see Supporting Information, Figure S2).
geometry of the system, which in the case of a small spherical particle is described by eq 1. The effects of the carrier and lattice temperatures on the dielectric function also depend on the detailed band structure of the material and on the energy of the probe photon. It is well accepted that Cu2xSe behaves as a p-type degenerate semiconductor with a partially filled valence band.26,39 We report in Figure 3a a tentative and qualitative sketch of the band structure deduced from spectroscopic investigations reported in the literature for cubic berzelianite Cu2‑xSe. According to Al-Mamun et al.,42 the Cu2xSe system with x = 0.15 exhibits two interband optical transitions, a direct transition at 2.1 eV and an indirect transition at 1.3 eV. It is thus expected that the pump photon at 1.19 eV energy is absorbed by an intraband process alone similarly to what happens in noble metals in the near-infrared, and no indirect interband transitions are expected to occur. The subsequent heating of the carrier gas results in a smearing of the Fermi distribution and gives rise to a modulation of the interband transition probability for the probe light at 1.38 and 0.95 eV (Figure 3a), which results in a modulation Δε∞ of the ε∞ parameter in the Drude dielectric function of eq 2. Given that the largest contribution to ε∞ at the probed wavelength originates from the direct interband transitions, having an edge at much lower wavelengths, we understand that
ð5Þ
where Γ0 is the room temperature carrier damping and β a constant parameter. The effect on the extinction cross section is illustrated in Figure 3b. Comparing experimental results with numerical computation from quasi-static formulas of eqs 1 and 2, we estimated β = 1.8 1012 rad s1 K1 (see Supporting Information, Figure S2). The previous discussion provides a qualitative understanding of the dynamics observed in the experimental data reported in Figure 2. The increase of the carrier temperature TC leads to a red-shift of the plasmonic resonance through the increase of the real part of the dielectric function ε∞, whereas the increase of the lattice temperature TL leads to a broadening of the plasmonic resonance, through the increase of the free carrier damping. The red shift of the plasmonic resonance enhances (reduces) the transmissivity of the probe which is in the blue tail (red tail) of the extinction spectrum (Figure 2a,b). The broadening of the spectrum enhances the transmissivity for probe wavelengths in the central part of the spectrum and reduces it both in the blue as well as in the red tails (cf. Figure 3b). Clearly, the fast decay of the differential transmission occurring within the first few picoseconds after the excitation is related to the fast decrease of the carrier temperature, while the following tail, lasting for several hundreds of picoseconds, is related to the slower cooling of the lattice. In order to draw a quantitative comparison with the experiment, the theoretical differential transmission was computed as ΔT/T = exp(ΔσENL) 1, ΔσE being the variation attained by the extinction cross section according to the quasi-static formulas of eq 1, and the temperature-dependent dielectric function of Cu2xSe given by εðTC , TL Þ ¼ ε∞ þ Δε∞ ðTC Þ þ i
ω2
ω2P ΓðTL Þ ωðω2 þ Γ2 ðTL ÞÞ
ω2P þ Γ2 ðTL Þ ð6Þ
with TC and TL provided by numerical solution of the TTM, and G, η, and GL for the long time-scale dynamics, as fitting parameters. In doing so, we assumed a heat capacity of the lattice CL = 2.72 106 J m3 K1, according to the experimental data reported in ref 41 for α-Cu2Se, whereas the heat capacity constant of the carriers was estimated as γ = rγAu = 3.2 J m3 K2, with γAu = 63 J m3 K2 being the heat capacity constant of the free electrons in Au and r the ratio between Cu2xSe carrier density (estimated, from the plasma frequency 4714
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Figure 4. (a) Carrier (hole) and lattice temperature numerically computed from the TTM of eq 3. (b) Measured absorption (at 1045 nm pump wavelength) as a function of the incident pulse fluence showing saturation starting from few hundred μJ/cm2. (c) Interband dielectric permittivity Δε∞ = ηTC2. Fitted parameters: G = 1.2 1016 W m3 K1; GL = 2 1016 W m3 K1; η = 7 109 K2.
and hole effective mass at room temperature, to be NC = ωP2ε0mh/ e2 = 3 1021 cm3) and Au carrier density (5.9 1022 cm3). Given the incident pump fluence F, the absorbed pump power density PA(t) to be employed in the TTM calculations was estimated according to the formula ! rffiffiffiffiffiffiffiffiffiffiffiffiffi 4 lnð2Þ F 4 lnð2Þt 2 AðFÞ PA ðtÞ ¼ exp π NVLT0 T0 2 ð7Þ where V is the volume of the spherical nanoparticle, T0 = 150 fs is the pump pulse duration (fwhm), and A(F) is the measured pump absorption, reported in Figure 4b. Note that despite of the 92% absorption estimated from the low power continuous wave transmission measurements (corresponding to the 1.1 peak optical density reported in Figure 1), pump pulse excitation with femtosecond laser beams induces saturation of the absorption starting from pump fluences as low as a few hundred μJ/cm2. The numerical solution of the TTM model with the best fitting parameters is reported in Figure 4a, whereas the corresponding numerically computed differential transmissions are shown in panels a and b of Figure 2 (for the picosecond time scale at 900 and 1300 nm, respectively) and in Figure 2c (for the long time scale at 900 nm). From the numerical solutions we extracted a carrierphonon coupling factor G = 1.2 1016 W m3 K1. It
must be noted that this estimation is representative of the high temperature range of a thousand kelvin explored in the experiments and provides a G factor 1 order of magnitude lower than in Au in the same temperature range.44 Since the G factor is proportional to the carrier density,44 this lower G is in agreement with the lower carrier density in Cu1.85Se compared to Au. We also estimated η = 7 109 K2, see eq 4, which is about 5 times lower than what is found for silver structures (films) at 900 nm probe wavelength.36 The numerical simulation is in good agreement with the experiment at low pump fluences, while at the highest fluence of 4.45 mJ/cm2 it disagrees for the short time-scale dynamics (compare solid and dotted red curves in Figure 2a). Actually, for such a high fluence a large carrier excess energy is produced, as shown by the large effective carrier temperature predicted with the TTM, approaching 104 K (see red curves in Figure 4a). In these conditions, similarly to what happens in noble metals, the nonthermalized carriers, initially excited by the intense pump beam, start significantly contributing to the optical response of the system within the first few hundred femtoseconds of the dynamics, which is not well described with the empirical formula used in this work, reported in eq 4. At long delays, the significant residual optical response results from a larger dependence of the Drude broadening Γ on the lattice temperature compared to metals. Gorbachev and Putilin, for thin films of similar composition and in the same temperature range explored in our experiments, found a strong red-shift of the plasma frequency with increasing lattice temperature.27 This shift was attributed to a large increase of the average carrier mass with increasing lattice temperature (based on the assumption that the carrier density remained constant, despite no direct measurement of the carrier density with temperature was reported). This must be in turn related to a change of the electronic band structure, thus of the crystal structure, which is is well-known to feature a complex phase diagram for copper selenide. In particular, in the case of bulk films Cu2xSe can crystallize in a variety of phases, i.e., cubic, tetragonal40,41 (although the tetragonal phase is rarely reported for Cu:Se stoichiometry deviating from 2:1), and monoclinic. In this respect, with increasing temperature in a range around 300360 K the crystal first undergoes a second-order transition from the low-temperature “ordered” monoclinic phase to a partially disordered cubic phase and then a first-order transition from this phase to the cubic disordered phase.4 Here the progressive loss of order is referred to that of vacancies and Cu occupancies in the various sites/sublattices. The transitions have also been identified in marked changes of the electronic and superionic conductivity,45,46 the former change clearly related to a change of the band structure. Since these phase transitions occur in a temperature range that is partially probed by our experiments on the Cu2xSe nanoparticles discussed here, we recorderd a series of X-ray powder diffraction patterns on a sample of these nanocrystals (kept under vacuum to prevent oxidation), in a temperature range from room temperature up to 200 C (473 K). All the recorded patterns, also at room temperature, clearly indicated that the nanoparticles have cubic phase (see Supporting Information, Figure S1, for details) and no phase transition is observed. Therefore, we conclude that the absence of red shift of the plasma frequency in our experiment could be related to the absence of the higher temperature first-order transition for the nanoparticles, compared to the bulk films. Nevertheless, we expect that even in nanocrystals the underlying increase with temperature of the 4715
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Nano Letters disorder in one of the copper/vacancy sublattice is still occurring and that it relates to the large temperature coefficient β of the plasma broadening found in our experiments. Indeed, this broadening is certainly dominated by the scattering of carriers on vacancies and other defects. In conclusion, we have reported the first observation of the ultrafast optical response of colloidal Cu2xSe nanocrystals excited at the plasmon resonant energy. We have been able to describe quantitatively the optical response and to extract from it the electronphonon coupling constant, which compares reasonably well with those found in noble metals after accounting for the lower free carrier density. We remark that the relatively low carrier density and structural peculiarities of Cu1.85Se lead to significant deviations of the response from the well-known behavior found in the more traditional metallic nanoparticles (for example the noble metal nanoparticles). In particular, the lower carrier density is responsible for a smaller carrier heat capacity and for a much higher effective carrier temperature at comparable fluences. This leads to strong nonlinear effects at high fluences both during the pump absorption process and during the carrier temperature relaxation process. Moreover, we also found a peculiar, strong dependence of the broadening of the plasmonic resonance with the lattice temperature which is presumably related to the underlying disordered structure of these materials. On the basis of the results from the present study, indicating a giant nonlinear response exceeding 40% relative transmission changes, one possible technological application of these novel plasmonic nanoparticles could consist for example in the development of ultrafast nanodevices operating at terahertz modulation bandwidth,30 having potential interest for telecom applications, with the peculiar advantages of plasmon resonance tunability in the near-IR region and ease of processing.
’ ASSOCIATED CONTENT
bS
Supporting Information. X-ray diffraction patterns and absorption spectra of the Cu2-xSe nanocrystals at different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Author Contributions #
F. S., G.D.V., and A.R.S.K. equally contributed to this work.
’ ACKNOWLEDGMENT The authors acknowledge financial support through the FP7 ERC starting grant NANO-ARCH (Contract No. 240111) as well as through the projects PITNGA-2009-237900(ICARUS) and FP7-ICT-248052(PHOTOFET). ’ REFERENCES (1) Odom, T. W.; Schatz, G. C. Chem. Rev. 2011, 111 (6), 3667. (2) Maier, S. A., Plasmonics: Fundamentals and Applications; Springer: New York, 2007. (3) Tam, F.; Goodrich, G. P.; Johnson, B. R.; Halas, N. J. Nano Lett. 2007, 7 (2), 496–501. (4) Korzhuev, M. A.; Baranchikov, V. V.; Abrikosov, N. K.; Bankina, V. F. Sov. Phys. Solid State 1984, 26, 1341.
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