Electrochemical Evaluation of Dopant Energetics and the Modulation

Article pubs.acs.org/JPCC

Electrochemical Evaluation of Dopant Energetics and the Modulation of Ultrafast Carrier Dynamics in Cu-Doped CdSe Nanocrystals Sourav Maiti,†,‡ Jayanta Dana,† Yogesh Jadhav,‡ Tushar Debnath,† Santosh K. Haram,*,‡ and Hirendra N. Ghosh*,†,§ †

Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Institute of Nano Science and Technology, Mohali, Punjab 160062, India ‡ Department of Chemistry, Savitribai Phule Pune University, Ganeshkhind, Pune 411007, India §

S Supporting Information *

ABSTRACT: Cyclic voltammetric and femtosecond transient absorption (TA) measurements on Cu+-doped CdSe nanocrystals (NCs) were utilized to reveal the energetics of the electroactive Cu+ dopant with respect to the band energies of CdSe NC host and the influence of Cu in tuning the carrier dynamics, respectively. Oxidation−reduction peaks due to an electroactive dopant within CdSe NC host have been traced to determine its energy level which was correlated to the dopant emission energy and Stokes shift. The low doping density of Cu does not significantly alter the band structure of CdSe as the shape of the TA spectra remains similar before and after doping. However, Cu+ acts as a hole localizing center decoupling the electronic wave function from the hole leading to slower Auger-assisted electron cooling in doped NCs. As hole localization to Cu+ is the primary step for dopant emission, in the presence of hole quenchers (aminophenols) the dopant emission gets drastically quenched. Interestingly, once hole is captured by Cu+ due to strong affinity for electron, external quenchers (nitrophenols) are unable to capture the electron as confirmed from steady state and time-resolved measurements establishing the role of Cu as an internal sensitizer for the charge carriers.

1. INTRODUCTION Doping transition metal ions in a semiconducting nanocrystalline host efficiently introduces new optoelectronic properties resulting from dopant−exciton interaction.1−7 In this regard, Cu-doped semiconducting NCs are promising candidates for light emitting devices and display technologies due to tunability of emission, long radiative lifetimes with high quantum yields, and large Stokes shift.4,8−19 In Cu-doped NCs, a broad tunable dopant photoluminescence (PL) arises from the recombination of delocalized conduction band (CB) electron with the Culocalized hole. The photophysical properties of Cu-doped NCs have been extensively studied including tunability of the dopant emission, single particle spectroscopy, electronic structure of doped NCs revealed through density functional theory (DFT) calculations, and use of Cu dopant as nanosensor to monitor the optoelectronic properties of NC host.4,9−11,18−23 However, electrochemical measurement on doped NCs where the signal due to an electroactive dopant is traced to determine the energetics of the dopant relative to the NC host is rare. Electrochemical measurements based on cyclic voltammetry (CV) are one of the simplest ways to determine the band-edge parameters and trap along with midgap state energetics of NCs.24−27 Considering this, the present study presents cyclic voltammetric measurements on Cu-doped CdSe © 2017 American Chemical Society

NCs explicitly determining the dopant energy level and thus correlating the large Stokes shift and dopant emission energy with band-edge energies of CdSe NC host. Previously, few attempts were made for bulk materials in the case of bulk ZnSe, CdSe, CdS, and CdTe crystals and in CdSe NCs through sophisticated ultraviolet photoelectron spectroscopy (UPS); however, determination of energy levels for the Cu dopant in the NC host through electrochemical technique still remains elusive.9,28−31 Recently Fuhr et al. have determined the Curelated defect state energy in CIS NCs through electrochemistry correlated with the photoluminescence Stokes shift.32 Moreover, the role of Cu dopant in affecting the exciton dynamics in ultrafast time scale for Cu-doped II−VI NCs has not been elucidated in detail so far. Earlier Gul et al. have reported charge carrier dynamics in nanosecond time scale for Cu-doped ZnSe NCs through measuring the effect of Cu doping in the luminescence decay of the band-edge emission.33 However, to fully explore the optoelectronic properties, the exciton dynamics needs to be investigated in picosecond to femtosecond time scale. The influence of Cu on the exciton Received: October 16, 2017 Revised: November 7, 2017 Published: November 8, 2017 27233

DOI: 10.1021/acs.jpcc.7b10262 J. Phys. Chem. C 2017, 121, 27233−27240

Article

The Journal of Physical Chemistry C

redispersed in chloroform. The temperature was maintained at 170 °C as black CuSe formed at higher temperatures. The Cu precursor (Cu-SA) is a Cu2+ complex, but TOP can reduce Cu2+ to Cu+ under inert N2 atmosphere during the synthesis.17,21,34,35 c. Cyclic Voltammetric Measurements. The cyclic voltammetric measurements were performed with the help of a Biologic SP-300 Potentiostat/Galvanostat. A commercial Au disk electrode (CHI Instruments, U.S.A., 2 mm diameter), silver wire, and platinum wire loop were used as working, quasireference, and counter electrodes, respectively. Before the experiment, all three electrodes were cleaned using dilute nitric acid. Especially Au disk working electrode was polished over 0.5 μm Al2O3 powder and rinsed with copious amounts of DI water. It was then further electrochemically cleaned by potentiodynamically cycling in 0.5 M H2SO4 between the potential range 1.5 and −0.5 V (scan rate of 1000 mV/s). As CV is a very sensitive technique, before CV measurement it is very necessary to clean the working electrode. If the surface of the working electrode is contaminated, it will show unwanted peaks that will spoil the experiment. Even after polishing the Au disk electrode with 0.5 μm alumina powder, the electrode is potentiodynamically cycled at high scan rate (i.e., 1 V) until the characteristic peaks for Au are observed to clean the surface and ensure it is cleaned. To avoid the interference of atmospheric CO2 and moisture, the measurements were carried out in a homemade vacuum electrochemical cell, with a special provision to transfer the solvent. Then 0.341g of TBAP (typically, 100 mM stock 10 mL solution) was transferred first in the cell and vacuum-dried in situ, at 80 °C for 2 h. The cell was then cooled to room temperature and brought to atmospheric pressure by relieving the vacuum through high purity nitrogen gas. Then 10 mL of DCM (dried in CaH2 overnight, double distilled, and stored in molecular sieves) was injected into the cell through a silicone-septum under the nitrogen atmosphere. For the controlled measurements, blank CV was recorded in the TBAP−DCM mixture without any sample. After that, the dispersion of quantum dots (1 mg/mL), 0.2 mL, was introduced. At the end of each set of experiments, the potentials were calibrated with respect to the normal hydrogen electrode (NHE), using ferrocene as an internal standard.36 For NC cyclic voltammetry measurements, a scan rate of 100 mV/s was used. d. Transient Absorption Spectroscopy (TAS). For TA measurements, the NCs were excited with 400 nm pump beam (fwhm ∼120 fs), and the electron population−depopulation of the excited state was probed in the entire visible region. The pump fluency was kept low enough (not exceeding 0.5 μJ), and the number of exciton/NC was kept below 0.5 to avoid multiexcitonic effects. Detailed calculations are shown in the Supporting Information. The pump beam was generated by frequency doubling of 800 nm amplified laser in a β-barium borate (BBO) crystal at 1 kHz repetition rate. The seed pulse from Ti-sapphire laser oscillator (Tissa 50, CDP, Russia) was amplified through multipass amplification technique pumped by 20W DPSS laser (Jade-II, Thales Laser, France). The broadband probe light results from the focusing part of 800 nm light (1 μJ) on a sapphire window. An imaging spectrometer (CDP2022i) was used as a detection system. Further details of the experimental setup are given elsewhere.37

dynamics will be particularly interesting to investigate as doping Cu in CdSe introduces states closer to the valence band (VB) which selectively localizes the hole making the electron and hole act differently. To shed light on the fundamental aspects of energetics and dynamics, Cu+-doped CdSe NCs were synthesized in the low doping regime using colloidal hot injection method with longlived red-shifted Cu-related emission. Distinct oxidation and reduction features were identified in the cyclic voltammogram due to electroactive Cu dopant along with the CdSe band-edge peaks. The Cu level was determined to be at ∼5.09 V in the CdSe NC host which is very close to values reported for Cu in the bulk semiconductors.30−33 The emission maximum of 1.63 eV for the Cu-related emission was nicely correlated with the difference between conduction band-edge of CdSe NC and Cu level. Moreover, the shape of the TA spectra remains unaltered upon Cu doping as the low Cu concentration did not severely affect the electronic structure of the CdSe NC. Interestingly, Auger-assisted electron cooling gets slower with increase in Cu concentration which can be explained in terms of decoupling the electronic wave function from hole since the Cu dopant acts as an intrinsic hole trapping center. If the hole is quenched at the first place with aminophenols, the dopant emission gets quenched; however, once the hole gets transferred to Cu+ activating it to Cu2+, then the Cu has a strong affinity for electrons, and electron transfer to external electron quencher such as nitrophenols gets restricted.

2. EXPERIMENTAL SECTION a. Materials. Cadmium acetate dihydrate (CdAc2·2H2O, 98%), selenium powder (Se, 99.99%), oleic acid (OA, 90%), trioctyl phosphine (TOP, 90%), stearic acid (SA, 95%), and octadecene (ODE, 90%) were purchased from Sigma-Aldrich. Copper chloride dihydrate (CuCl2·2H2O, 99%) and tetramethylammonium hydroxide pentadydrate (TMAH, 98%) were purchased from Alfa Aesar. All the chemicals were used as is without further purification. In order to clean the NCs, AR grade chloroform and AR methanol were used. Spectroscopic grade chloroform was used for transient absorption measurements. Tetrabutylammonium-perchlorate (TBAP), DCM, and ferrocene were purchased from Sigma-Aldrich for CV measurements. b. Synthesis of Cu-Doped CdSe NCs. Cu-doped CdSe NCs have been synthesized using synthetic methods reported in the literature with required modifications.8,21 At first, Custearate was synthesized from CuCl2 and SA following procedures of Srivastava et al.17 Briefly, 5 mL, 2.9 g of 25% methanolic TMAH solution was added dropwise to 10 mmol SA dissolved in 20 mL of methanol at 50 °C while stirring. After 30 min, 4 mmol of CuCl2 in 10 mL of methanol was dropwise added to the solution. The resulting sky-blue precipitate of copper stearate (Cu-SA) was separated, washed with methanol, and dried under vacuum. In a typical synthesis of Cu-doped CdSe NCs, 1 mmol of CdAc2·2H2O, 0.1 mmol of Cu-SA (mole ratio 1:10), and 4 mmol of OA was heated to 180 °C in 10 mL of ODE under nitrogen atmosphere. At 180 °C, 0.25 mmol of Se-TOP in 1 mL of ODE was rapidly injected into the reaction mixture. The reaction was continued for 20 min at 170 °C to ensure incorporation of Cu. In this method the Cu precursor remains present when the nanocrystals nucleate in the solution leading to incorporation of Cu within the NC. Once the reaction was complete, the NCs were washed with methanol thrice to remove all the excess precursors and 27234


Article


3. RESULTS AND DISCUSSION Cu-doped CdSe NCs (termed as Cu−CdSe) were synthesized by following previously reported procedure with required modifications.8,17,21 Two different doping concentrations of Cu were obtained by varying the amount of Cu precursor; Cu− CdSe(1) and Cu−CdSe(2) with 0.66% and 2% of Cu as determined from elemental analysis through inductively coupled plasma mass spectrometry (ICP-MS). This translates to ∼1.6 and ∼5.3 Cu dopant atom per NC on average which is on the low doping regime where there will be no interaction between the dopants. Undoped CdSe NCs were also synthesized under similar conditions as a reference sample. The sizes of the NCs were calculated from the first excitonic maxima of the NCs using the CdSe sizing curve and determined to be 2.91 and 3.12 nm for Cu−CdSe(1) and Cu−CdSe(2), respectively.38 The size determined from transmission electron micrographs of Cu−CdSe(2) is 3.14 ± 0.15 nm (Figure S1A) in close agreement with the size determined from the sizing curve of CdSe. EPR measurement suggests the presence of Cu1+ as paramagnetic signal originating from when Cu2+ was absent (Figure S1B). Therefore, we can conclude that the Cu(II)-stearate precursor gets reduced in the presence of TOP under nitrogen atmosphere.17,21,34,35 EPR has been extensively used as evidence for the presence of Cu+ in doped NC for similar Cu-doping concentrations.4,17,21,33,34,39 The X-ray diffraction (XRD) pattern also shown in Supporting Information Figure S1C indicates zinc blend structure formation in accord with the literature.8,11,23,33 Figure 1 shows the absorption and emission spectra of Cu− CdSe(2) NCs in comparison with undoped CdSe. In undoped CdSe the first excitonic absorption peak (Figure 1a) appears at ∼550 nm (2.25 eV) with a band-edge emission maxima (Figure 1b) at ∼575 nm (2.16 eV). On the other hand, the doped NCs have a sharp first excitonic absorption (Figure 1c) at ∼555 nm (2.23 eV) with a broad red-shifted emission (Figure 1d) centered at ∼760 nm (1.63 eV) along with a weak band-edge emission (Figure 1e) maxima at ∼570 nm (2.18 eV). The excitation spectra measured for 760 nm emission closely matches with the absorption spectra (Figure 1f) implying the broad red-shifted emission involves CdSe band gap transition. This broad emission has been attributed to the metal to ligand (conduction band) charge transfer transition (MLCBCT) involving the recombination of conduction band (CB) electron to the Cu-localized hole. The exact mechanism of this transition depends on the oxidation state of Cu.4,20,39−42 This broad emission involving Cu states gets quenched in the presence of dodecanethiol (DDT) further suggesting the presence of Cu+ as a dopant (Figure 1g).14 Moreover, the broad weaker absorption “foot” tailing to longer wavelengths after the first excitonic 1S peak (Figure 1c) corresponding to direct photoexcitation of the MLCBCT absorption band is also a characteristic feature of Cu+-doped CdSe NCs.8,43 For Cu2+ (d9 system) the CB electron directly recombines with the optically active hole in Cu resulting in the Cu-related emission. Conversely, Cu+ doping represents a filled shell d10 system which needs to be oxidized in order to accept electrons. After photoexcitation electron−hole pair is generated in the CdSe host. The hole from the VB rapidly (in picosecond time scale) localizes in Cu+ oxidizing it to Cu2+ followed by recombination of an electron from CB with the hole in the Cu2+.4,21 This converts the Cu2+ again to the original Cu+ center capable of accepting the hole.

Figure 1. UV−vis absorption (a) and emission (b) of undoped CdSe. UV−vis absorption (c), Cu-related emission (d), band-edge emission after 10× zoom (e), and excitation spectra measured at dopant emission maxima (f) for Cu−CdSe(2) NCs. The absorption foot due to direct excitation of MLCBCT band in Cu−CdSe(2) is shown separately. (g) Cu-related emission spectra in the presence of dodecanethiol (DDT). The emission gets quenched in the presence of DDT suggesting the dopant is in +1 oxidation state (Cu1+).14 If Cu was in +2 state the emission should have increased as pointed out by Viswanatha et al.14 (Inset) Emission decay traces for band-edge emission in undoped CdSe (h). In the case of Cu−CdSe(2), the decay traces for Cu-related emission at 760 nm (i) and band-edge emission at 570 nm (j).

Since in the Cu-doped CdSe the CB electron delocalized over the whole nanocrystal recombines with a hole localized in the Cu center, the Cu emission has a very long lifetime (300− 500 ns) compared to CdSe band-edge emission.4,8,9,14,40 In our case the exciton lifetime (τPL) of pure CdSe (Figure 1h) was found to be 12.1 ns (1.1 ns, 35%; 7.8 ns, 36%; 30.4 ns, 29%) whereas the τPL at 760 nm for Cu−CdSe(2) was single exponential (Figure 1i) having τPL of ∼350 ns which is in accord with the literature mentioned above. Interestingly, the weak band edge emission in the doped NC gets quenched (Figure 1j) with τPL of 0.69 ns (0.27 ns, 84% and 2.98 ns, 16%) due to fast hole localization in Cu from the VB of CdSe. To find out the hole transfer time accurately, luminescence upconversion measurements were carried out monitoring the PL decay at 570 nm (Figure S2, Supporting Information). The appearance of an additional fast ∼600 fs component for Cu− CdSe(2) can be attributed to hole transfer from VB of CdSe to Cu center. This time scale is analogous to the flattening distortion of [Cu(dmphen)2]+ associated with the hole transfer from ligand π* orbital to Cu+.44,45 As Cu is a well-known electroactive material, we investigated if Cu states have any role to play in cyclic voltammetric measurements. We compare both the Cu−CdSe NCs and undoped CdSe of similar size as represented in Figure 2 and Supporting Information Figure S3. For CdSe prominent oxidation (anodic) and reduction (cathodic) peaks are observed at −5.54 and −3.34 V (vs local vacuum), respectively (Table 1). For Cu−CdSe(1) the anodic and cathodic peaks are at very similar positions of −5.52 and −3.28 V, respectively. 27235


Article


Supporting Information). The Cu level in the case of bulk semiconductors has been reported to be ∼0.65 eV above the VB level of CdSe,30 ∼0.68 eV above the VB in ZnSe,29 at ∼0.15 eV over the VB in CdTe,31 and at 1.2 eV above the VB in CdS.28 The compilation of the literature values as depicted in Supporting Information Figure S4 translates to ∼5.1 (±0.05) eV for the Cu-related atomic-like state which is in very close agreement with the value determined from CV experiments. Our measured potential match reasonably well with the reported redox potential of Cu1+/2+ couple under different chemical environments46−48 and with the recently reported energy of the Cu defect state (∼5.54 eV) in copper indium sulfide (CIS) NCs.32 Moreover, the difference of 1.64 V between the CB edge (−3.38 V) and Cu level (−5.02 V) is in good agreement with the emission maxima of 1.63 eV for Curelated emission in Cu−CdSe(2). The Stokes shift of 0.6 eV is also correlative with the difference between VB edge (−5.53 eV) and Cu state (−5.02 eV) of 0.51 eV. Ultrafast transient absorption (TA) measurement was utilized to reveal the role of Cu dopant in the exciton dynamics after exciting the NCs dispersed in chloroform with 400 nm pulsed laser (fwhm ∼120 fs) and probing the photoinduced changes in the absorption (ΔOD = ODpump on − ODpump off) in the entire visible region. Recently, Yang et al. have utilized TAS to investigate the exciton dynamics in heavily Cu-doped III−V low band gap InAs NCs where the number of Cu per NC varies from 12 to 500 and the role of Cu is fundamentally different from II−VI semiconductors.49 In that case the density of dopant-related states increases near the CB-edge resulting an impurity sub-band at significantly higher doping concentrations. Here, we have employed TAS to reveal the role of Cu dopant on the exciton dynamics in II−VI CdSe NCs in the low doping regime where Cu-related single atomic-like state appears to be within the band gap of CdSe NC. Figure 3A shows the TA spectra at different time intervals for CdSe and Cu-doped CdSe NCs. The shape of the TA spectra is very similar for both doped and undoped NCs which is quite expected as the doping does not drastically change the shape of the linear absorption spectra. The bleach around ∼555 nm is the ground state 1S excitonic bleach attributed to 1Se(e)−1S3/2(h) electronic transition. However, the bleach dynamics as depicted in Figure 3B manifested by the bleach growth and recovery is quite different in the doped NCs. The bleach growth and recovery kinetics essentially represents the population and depopulation of the electron in the CB state for II−VI semiconductors, and the contribution of hole in the bleach signal is insignificant due to higher density of hole states dispersing the hole population over many adjacent levels.50,51 For undoped CdSe the bleach growth has pulse width limited (400 ps component is due to radiative recombination and nonradiative electron trapping process. All the multiexponential fitting parameters are tabulated in Table S1, Supporting Information. As the NCs are excited with energy (400 nm, 3.1 eV) higher than the band-edge of the NCs (∼555 nm, 2.23 eV), the photoexcited hot electron cools down to band-edge state (path q, Figure 3C) within the first few picoseconds manifested by the bleach growth. In CdSe NCs the CB electronic states are apart by hundreds of meV which is quite large compared to typical phonon energies of ∼20 meV.50,52,53

Figure 2. Cyclic voltammograms of (a) CdSe and (b) Cu−CdSe(2) NCs. The arrows denote the anodic and cathodic current, respectively, due to the Cu dopant. The red dotted arrow denotes the VB-edge peak.

Table 1. Conduction Band Edge, Valence Band Edge of CdSe, Cu−CdSe(1) and Cu−CdSe(2) NCsa size (nm)

optical band gap (eV)

CB-edge (V) vs vac/(vs NHE)

VB-edge (V) (vs vac/vs NHE)

CdSe

3.0

2.25

Cu− CdSe(1) Cu− CdSe(2)

2.91

2.28

3.12

2.23

−3.34/ (−1.16) −3.28/ (−1.22) −3.38/ (−1.12)

−5.54/ (1.04) −5.52/ (1.02) −5.53/ (1.03)

sample

quasi particle gap (V)

Cu-peak (V) (vs vac/vs NHE)

2.20

−

2.24

∼−5.16/ (0.66) ∼−5.02/ (0.52)

2.15

a

For Cu−CdSe the Cu-related values are also given. Values are with respect to both vacuum and NHE.

Similarly for Cu−CdSe(2) the anodic and cathodic peaks are observed at −5.53 and −3.38 V, respectively. The anodic peak potential corresponds to the removal of electron from the VBedge, and the cathodic peak potential represents addition of electron to the CB-edge of NCs. The determined VB- and CBedges for both CdSe and Cu−CdSe match reasonably well with the literature as compared in Figure S4, Supporting Information.27 For Cu−CdSe(1) a prominent peak at ∼−5.16 V was observed (Figure S3, Supporting Information) which we attribute to Cu-oxidation peak as it was not present in the undoped CdSe. Therefore, the Cu-related state is located at ∼5.16 eV. Interestingly, Cu−CdSe(2) shows additional oxidation−reduction peaks corresponding to ∼−5.16 and ∼− 4.88 V, respectively in the CV data as shown in Figure 2b. The anodic peak at ∼−5.16 V and cathodic peak at ∼−4.88 V are again attributed to the oxidation of Cu+ dopant to Cu2+ followed by the subsequent reduction of Cu2+ to Cu+. Therefore, we attribute that the Cu-related level has energy of (Eox+Ered)/2 = −5.02 V (Table 1). The position and intensity of the band-edge peaks and Cu-related peaks are independent of scan direction (Figure S5, Supporting Information) confirming credibility of the positions determined. The anodic oxidation peak due to Cu is more intense compared to the cathodic reduction peak mostly due to irreversibility of this redox cycle within the CdSe NC host. Comparing Cu−CdSe(1) and Cu−CdSe(2), the Cu-related state has energy ∼5.09 eV on average. The UV−vis absorption spectra remain very similar before and after CV experiments suggesting negligible degradation of the NCs (Figure S6, 27236


Article


Figure 3. (A) TA spectra for (a) CdSe, (b) Cu−CdSe(1), and (c) Cu−CdSe(2) upon 400 nm photoexcitation. (B) Kinetics of ground state (1S) bleach recovery for (d) CdSe, (e) Cu−CdSe(1), and (f) Cu−CdSe(2). There is break between 2 and 2.01 ps in the figure. (C) Schematically depicting all the processes in undoped and Cu-doped CdSe. By monitoring at the 1S bleach position we are looking at the population and depolulation of the electron in the 1S state. Upon photoexcitation (p) electron−hole pair is generated, and the electron resides in an upper excitonic state as the pump energy (3.1 eV) is higher than 1S transition (2.25 eV). The electron dissipates its excess energy through Auger-assisted cooling (q) which depends on electron−hole wave function overlap. The electron cooling is depicted through the growth of the 1S bleach. Once the electron reaches the 1S state, the electron can depopulate (reflected by the recovery of the bleach) through radiative recombination (r) or trapping (s). In the case of Cu-doped CdSe due to fast hole transfer (∼600 fs) to Cu+ (t), the CB electron gets decoupled from hole resulting in slower electron cooling. After hole transfer the oxidized Cu2+ accepts the CB electron to have the characteristic dopant PL. Therefore, the Cu creates an additional pathway (u) for electron depopulation from the 1S state, leading to marginally faster recovery of the 1S bleach. As the electron transfer to Cu from CB is a slower process, the recovery becomes slightly faster at longer time scale.

recombination with hole (path r and s, Figure 3C). In Cudoped CdSe the Cu center upon capturing the hole recombines with the CB electron providing an extra pathway for electron depopulation (path u, Figure 3C). As the electron (CB)−Cu2+ recombination is a slow process, the recovery becomes marginally faster at longer time scales. Following this the aim of the current investigation is to decipher the effect of Cu+ center on the charge transfer dynamics to an external molecule with different anchoring groups. As stated earlier, in the Cu+ oxidation state, Cu acts as a hole trapping center located intrinsically within the NC. Therefore, it will be interesting to see the effect of external molecules, either electron or hole acceptors, on the dopant PL. Two substituted phenols with electron-rich amino functional group (o-AP) and with electron withdrawing nitro group at the ortho position (o-NP) were chosen to elucidate their role in Cu-related emission. It is known in literature that depending on the energetics amino-substituted molecules act as hole quenchers whereas nitro-substituted molecules are typical electron quenchers for CdSe NCs.56−58 From CV measurements (Figure S7, Supporting Information) we also found that the lowest unoccupied molecular orbital (LUMO) of o-NP can energetically accept electrons, while the o-AP has suitable highest occupied molecular orbital (HOMO) level for capturing the hole from Cu−CdSe (Scheme 1). The absorption spectra of Cu−CdSe(2) remain similar upon addition of both the phenols as shown in Supporting Information Figure S8. Interestingly, for both the phenols the broad dopant emission gets quenched; however, the quenching

Therefore, cooling of excited electron is unfavorable through phonons. Subsequently, an Auger-assisted electron cooling mechanism was proposed and later experimentally verified where the electron cools down by transferring its energy to valence band hole which relaxes through dense phonon modes.52−54 This Auger-type energy transfer is strongly size dependent;55 however, as we are using NCs of similar size (∼3 nm) we can exclude any size dependence for the observed cooling process. Moreover, this electron−hole energy transfer process depends on the wave function overlap of both the charge carriers; if the charge carriers are decoupled, the electron cooling time increases.53,54 Once decoupled from the hole, the hot electron can relax through ligand vibrations. We also exclude the effect of surfactants or ligands in the relaxation process as both the doped and undoped NCs were synthesized under identical conditions. In Cu-doped NCs the Cu+ acts as a hole trapping center by capturing the hole and thus diminishing the electron−hole overlap integral. Therefore, as the amount of Cu increased we envision slower electron cooling of 0.5 ps in Cu−CdSe(1) and 0.7 ps in Cu−CdSe(2) due to decoupling of electron−hole wave functions. This is in contrast to heavily Cudoped InAs NCs where the hot electron relaxation became faster with doping concentration as the impurity sub-band near the CB facilitates electron relaxation.49 In Cu-doped CdSe, Cu introduces an atomic-like state near the VB of CdSe and the cooling mechanism is fundamentally different as discussed above. Moreover, the recovery of 1S bleach is marginally faster in the case of Cu−CdSe NCs. In the case of undoped CdSe the electron depopulates from the 1S state though trapping and 27237


Article


hand, in the case of electron quenchers the bleach recovery becomes faster due to fast depopulation of the CB electron to the LUMO of quencher.57,59,61 Interestingly, from TAS we do not find any evidence of electron transfer from doped CdSe to o-NP as the ground state bleach recovery remains similar in the presence of o-NP as shown in Figure 4B. This surprising behavior can be explained considering the mechanism of Curelated emission as depicted in Scheme 1. As the Cu dopant is in the +1 oxidation state, hole localization in Cu is the primary step for dopant emission. Therefore, once the hole is captured through o-AP, the dopant emission gets drastically quenched. On the other hand, once hole gets localized on Cu1+, the oxidized Cu2+ develops strong affinity toward the CB electron. Therefore, in the presence of electron quenching o-NP electron transfer does not occur. The quenching at higher concentrations occurs through dynamic quenching mechanism involving collision of the o-NP with NCs. Our findings are corroborative with Sarkar et al., who also observed very slow reduction of nitrophenol in Cu-doped ZnS NCs compared to undoped ZnS NCs.58 As the Cu dopant strongly interacts with the exciton, the transfer of CB electron to external quenchers get retarded. Therefore, in our investigation we have addressed two important aspects of energetics and dynamics in Cu-doped CdSe NCs. The Cu-dopant-related oxidation−reduction peaks in the cyclic voltammogram have been correlated to its exact energy level. Moreover, the role of Cu as an strong internal hole sensitizer has been established through TAS where electron cooling slowed down 1.75 times in the presence of 2% Cu. The hindrance of electron transfer to external electron quenchers further elucidates the role of Cu in the charge transfer dynamics.

Scheme 1. Band-Edge Positions and Cu Level of Cu− CdSe(2) As Determined from CV Experiments. The HOMO−LUMO of Ortho Aminophenols (o-AP) and Ortho Nitrophenols (o-NP) Relative to the Vacuum As Determined from Cyclic Voltammetry Experiments in the Context of Cu−CdSe(2)

propensity is quite different as depicted in the Stern−Volmer plots (Figure S8, Supporting Information). Between o-AP and o-NP, the later is a weak quencher as higher concentrations of o-NP is required to have the same quenching effect. To further confirm this trend, we have carried out time-resolved emission measurements (TCSPC) by monitoring the decay of Curelated emission (∼760 nm) in the presence of phenols which are shown in Figure 4A. For o-AP (0.25 mM) the PL decay

4. CONCLUSIONS In summary, Cu-doped CdSe NCs were synthesized, and the dopant was characterized as Cu+. The broad dopant-related emission with large Stokes shift was attributed to hole localization from the VB in the Cu+ state followed by capture of the CB electron as established in the literature. The optical properties were compared with the CV measurements where additional oxidation−reduction peaks arise due to the electroactive Cu dopant electrochemically confirming the Cu energy level within the doped NC. The Cu-related red-shifted emission energy was well correlated with the Cu level determined from CV measurements. Moreover, the hot electron cooling time to the 1Se state slows down with increase in Cu concentration due to localization of the hole in the Cu eventually reducing the electron−hole wave function overlap. In presence of external hole acceptors (aminophenols), the dopant emission gets quenched due to hole transfer; however, electron transfer was not operative with electron acceptors (nitrophenols). This surprising observation was attributed to the mechanism of Cu emission with Cu+ as a dopant and strong affinity of Cu toward electron once it is oxidized to Cu2+ with the VB hole. Thus, our study provides fundamental information about the electrochemistry and exciton dynamics of Cu-doped CdSe NCs which pave the way for better understanding of the dopant−exciton interaction.

Figure 4. (A) PL decay traces for Cu-related emission and (B) kinetics of the ground state bleach dynamics for (a) Cu−CdSe(2), in the presence of (b) o-AP, (c) o-NP.

becomes triexponential (0.44 ns, 74%; 7.87 ns, 13%; 69.2 ns, 13%) with an initial fast component of 0.44 ns indicative of thermodynamically viable hole transfer process to the HOMO of the o-AP. On the other hand, for o-NP even at very high concentrations (2.5 mM) the PL decay becomes biexponential (14.42 ns, 18% and >100 ns, 82%) without any appreciable quenching. This excludes minimum possibility of electron transfer to the LUMO of o-NP. The multiexponential fitting parameters for PL decay are shown in the Supporting Information (Table S2). To understand the paradoxical behavior of nitro- and aminophenols to quench the dopant emission and confirm charge transfer processes, TAS measurements were carried out in the presence of phenols. In case of hole quenching ligands, the ground state bleach recovery tends to become slower due to decoupling of hole from the electron.57,59−61 For o-AP we do envision slower bleach recovery indicating energetically favorable hole transfer from Cu−CdSe to HOMO of o-AP as shown in Figure 4B (the multiexponential fitting parameters are tabulated in Table S3, Supporting Information). On the other

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b10262. 27238


Article


■

(12) Pradhan, N.; Battaglia, D. M.; Liu, Y.; Peng, X. Efficient, Stable, Small, and Water-Soluble Doped ZnSe Nanocrystal Emitters as NonCadmium Biomedical Labels. Nano Lett. 2007, 7, 312−317. (13) Pandey, A.; Brovelli, S.; Viswanatha, R.; Li, L.; Pietryga, J. M.; Klimov, V. I.; Crooker, S. A. Long-lived Photoinduced Magnetization in Copper-Doped ZnSe-CdSe Core-Shell Nanocrystals. Nat. Nanotechnol. 2012, 7, 792−797. (14) Viswanatha, R.; Brovelli, S.; Pandey, A.; Crooker, S. A.; Klimov, V. I. Copper-Doped Inverted Core/Shell Nanocrystals with “Permanent” Optically Active Holes. Nano Lett. 2011, 11, 4753−4758. (15) Jana, S.; Srivastava, B. B.; Acharya, S.; Santra, P. K.; Jana, N. R.; Sarma, D. D.; Pradhan, N. Prevention of Photooxidation in BlueGreen Emitting Cu Doped ZnSe Nanocrystals. Chem. Commun. 2010, 46, 2853−2855. (16) Maity, A. R.; Palmal, S.; Basiruddin, S. K.; Karan, N. S.; Sarkar, S.; Pradhan, N.; Jana, N. R. Doped Semiconductor Nanocrystal Based Fluorescent Cellular Imaging Probes. Nanoscale 2013, 5, 5506−5513. (17) Srivastava, B. B.; Jana, S.; Pradhan, N. Doping Cu in Semiconductor Nanocrystals: Some Old and Some New Physical Insights. J. Am. Chem. Soc. 2011, 133, 1007−1015. (18) Pradhan, N.; Goorskey, D.; Thessing, J.; Peng, X. An Alternative of CdSe Nanocrystal Emitters: Pure and Tunable Impurity Emissions in ZnSe Nanocrystals. J. Am. Chem. Soc. 2005, 127, 17586−17587. (19) Pradhan, N.; Peng, X. Efficient and Color-Tunable Mn-Doped ZnSe Nanocrystal Emitters: Control of Optical Performance via Greener Synthetic Chemistry. J. Am. Chem. Soc. 2007, 129, 3339− 3347. (20) Nelson, H. D.; Li, X.; Gamelin, D. R. Computational Studies of the Electronic Structures of Copper-Doped CdSe Nanocrystals: Oxidation States, Jahn−Teller Distortions, Vibronic Bandshapes, and Singlet−Triplet Splittings. J. Phys. Chem. C 2016, 120, 5714−5723. (21) Whitham, P. J.; Knowles, K. E.; Reid, P. J.; Gamelin, D. R. Photoluminescence Blinking and Reversible Electron Trapping in Copper-Doped CdSe Nanocrystals. Nano Lett. 2015, 15, 4045−4051. (22) Grandhi, G. K.; Viswanatha, R. Demystifying Complex Quantum Dot Heterostructures Using Photogenerated Charge Carriers. J. Phys. Chem. Lett. 2017, 8, 2043−2048. (23) Grandhi, G. K.; Arunkumar, M.; Viswanatha, R. Understanding the Role of Surface Capping Ligands in Passivating the Quantum Dots Using Copper Dopants as Internal Sensor. J. Phys. Chem. C 2016, 120, 19785−19795. (24) Haram, S. K.; Quinn, B. M.; Bard, A. J. Electrochemistry of CdS Nanoparticles: A Correlation between Optical and Electrochemical Band Gaps. J. Am. Chem. Soc. 2001, 123, 8860−8861. (25) Kucur, E.; Riegler, J.; Urban, G. A.; Nann, T. Determination of Quantum Confinement in CdSe Nanocrystals by Cyclic Voltammetry. J. Chem. Phys. 2003, 119, 2333−2337. (26) Ma, X.; Mews, A.; Kipp, T. Determination of Electronic Energy Levels in Type-II CdTe-Core/CdSe-Shell and CdSe-Core/CdTe-Shell Nanocrystals by Cyclic Voltammetry and Optical Spectroscopy. J. Phys. Chem. C 2013, 117, 16698−16708. (27) Inamdar, S. N.; Ingole, P. P.; Haram, S. K. Determination of Band Structure Parameters and the Quasi-Particle Gap of CdSe Quantum Dots by Cyclic Voltammetry. ChemPhysChem 2008, 9, 2574−2579. (28) Heitz, R.; Hoffmann, A.; Thurian, P.; Broser, I. The Copper Centre: A Transient Shallow Acceptor in ZnS and CdS. J. Phys.: Condens. Matter 1992, 4, 157. (29) Grimmeiss, H. G.; Ovrén, C.; Ludwig, W.; Mach, R. Identification of Deep Centers in ZnSe. J. Appl. Phys. 1977, 48, 5122−5126. (30) Türe, I. E.; Claybourn, M.; Brinkman, A. W.; Woods, J. Copper Centers in CdSe. J. Appl. Phys. 1986, 60, 1670−1675. (31) Laurenti, J. P.; Bastide, G.; Rouzeyre, M.; Triboulet, R. Localized Defects in p-CdTe:Cu Doped by Copper Incorporation During Bridgman Growth. Solid State Commun. 1988, 67, 1127−1130. (32) Fuhr, A.; Yun, H. J.; Makarov, N. S.; Li, H.; McDaniel, H.; Klimov, V. I. Light-Emission Mechanism in CuInS2 Quantum Dots

Characterization of Cu−CdSe; up-conversion data; ICPMS analysis; additional CV data and CV of phenols; literature survey, S−V plots; table of multiexponential fitting parameters for TCSPC and TA data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; [email protected]. *E-mail: [email protected]. ORCID

Santosh K. Haram: 0000-0002-0618-1215 Hirendra N. Ghosh: 0000-0002-2227-5422 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by “DAE-SRC Outstanding Research Investigator Award” (Project/Scheme No. DAE-SRC/2012/ 21/13-BRNS) granted to H.N.G. We sincerely acknowledge Dr. R. M. Kadam in the Radiochemistry Division, BARC for EPR measurement, Dr. S. Nath, RPCD, BARC for upconversion experiments, and Abhijit Saha, RACD, BARC for ICP-MS measurements. S.M., J.D., and T. D. acknowledge CSIR for research fellowship. Y.J. acknowledge DST Inspire Ph.D. program for providing a research fellowship (Grant 2013/606).

■

REFERENCES

(1) Norris, D. J.; Efros, A. L.; Erwin, S. C. Doped Nanocrystals. Science 2008, 319, 1776−1779. (2) Erwin, S. C.; Zu, L.; Haftel, M. I.; Efros, A. L.; Kennedy, T. A.; Norris, D. J. Doping Semiconductor Nanocrystals. Nature 2005, 436, 91. (3) Beaulac, R.; Ochsenbein, S. T.; Gamelin, D. R. In Nanocrystal Quantum Dots; Klimov, V. I., Ed.; CRC Press: Boca Raton, FL, 2010; Vol. 2, p 397. (4) Knowles, K. E.; Hartstein, K. H.; Kilburn, T. B.; Marchioro, A.; Nelson, H. D.; Whitham, P. J.; Gamelin, D. R. Luminescent Colloidal Semiconductor Nanocrystals Containing Copper: Synthesis, Photophysics, and Applications. Chem. Rev. 2016, 116, 10820−10851. (5) Pradhan, N.; Sarma, D. D. Advances in Light-Emitting Doped Semiconductor Nanocrystals. J. Phys. Chem. Lett. 2011, 2, 2818−2826. (6) Debnath, T.; Maiti, S.; Ghosh, H. N. Unusually Slow Electron Cooling to Charge-Transfer State in Gradient CdTeSe Alloy Nanocrystals Mediated through Mn Atom. J. Phys. Chem. Lett. 2016, 7, 1359−1367. (7) Pradhan, N.; Das Adhikari, S.; Nag, A.; Sarma, D. D. Luminescence, Plasmonic, and Magnetic Properties of Doped Semiconductor Nanocrystals. Angew. Chem., Int. Ed. 2017, 56, 7038−7054. (8) Knowles, K. E.; Nelson, H. D.; Kilburn, T. B.; Gamelin, D. R. Singlet−Triplet Splittings in the Luminescent Excited States of Colloidal Cu+:CdSe, Cu+:InP, and CuInS2 Nanocrystals: ChargeTransfer Configurations and Self-Trapped Excitons. J. Am. Chem. Soc. 2015, 137, 13138−13147. (9) Grandhi, G. K.; Tomar, R.; Viswanatha, R. Study of Surface and Bulk Electronic Structure of II−VI Semiconductor Nanocrystals Using Cu as a Nanosensor. ACS Nano 2012, 6, 9751−9763. (10) Brovelli, S.; Galland, C.; Viswanatha, R.; Klimov, V. I. Tuning Radiative Recombination in Cu-Doped Nanocrystals via Electrochemical Control of Surface Trapping. Nano Lett. 2012, 12, 4372− 4379. (11) Grandhi, G. K.; Viswanatha, R. Tunable Infrared Phosphors Using Cu Doping in Semiconductor Nanocrystals: Surface Electronic Structure Evaluation. J. Phys. Chem. Lett. 2013, 4, 409−415. 27239


Article

The Journal of Physical Chemistry C Evaluated by Spectral Electrochemistry. ACS Photonics 2017, 4, 2425− 2435. (33) Gul, S.; Cooper, J. K.; Corrado, C.; Vollbrecht, B.; Bridges, F.; Guo, J.; Zhang, J. Z. Synthesis, Optical and Structural Properties, and Charge Carrier Dynamics of Cu-Doped ZnSe Nanocrystals. J. Phys. Chem. C 2011, 115, 20864−20875. (34) Corrado, C.; Jiang, Y.; Oba, F.; Kozina, M.; Bridges, F.; Zhang, J. Z. Synthesis, Structural, and Optical Properties of Stable ZnS: Cu,Cl Nanocrystals. J. Phys. Chem. A 2009, 113, 3830−3839. (35) Tananaev, P. N.; Dorofeev, S. G.; Vasil’ev, R. B.; Kuznetsova, T. A. Preparation of Copper-Doped CdSe Nanocrystals. Inorg. Mater. 2009, 45, 347−351. (36) Ding, Z.; Quinn, B. M.; Haram, S. K.; Pell, L. E.; Korgel, B. A.; Bard, A. J. Electrochemistry and Electrogenerated Chemiluminescence from Silicon Nanocrystal Quantum Dots. Science 2002, 296, 1293. (37) Maity, P.; Debnath, T.; Chopra, U.; Ghosh, H. N. Cascading Electron and Hole Transfer Dynamics in CdS/CdTe Core-Shell Sensitized Bromo-Pyrogallol Red (Br-PGR): Slow Charge Recombination in Type II Regime. Nanoscale 2015, 7, 2698−2707. (38) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals. Chem. Mater. 2003, 15, 2854−2860. (39) Car, B.; Medling, S.; Corrado, C.; Bridges, F.; Zhang, J. Z. Probing the Local Structure of Dilute Cu Dopants in Fluorescent ZnS Nanocrystals Using EXAFS. Nanoscale 2011, 3, 4182−4189. (40) Meulenberg, R. W.; van Buuren, T.; Hanif, K. M.; Willey, T. M.; Strouse, G. F.; Terminello, L. J. Structure and Composition of CuDoped CdSe Nanocrystals Using Soft X-ray Absorption Spectroscopy. Nano Lett. 2004, 4, 2277−2285. (41) Wright, J. T.; Forsythe, K.; Hutchins, J.; Meulenberg, R. W. Implications of Orbital Hybridization on the Electronic Properties of Doped Quantum Dots: the Case of Cu:CdSe. Nanoscale 2016, 8, 9417−9424. (42) Gul, S.; Cooper, J. K.; Glans, P.-A.; Guo, J.; Yachandra, V. K.; Yano, J.; Zhang, J. Z. Effect of Al3+ Co-doping on the Dopant Local Structure, Optical Properties, and Exciton Dynamics in Cu+-Doped ZnSe Nanocrystals. ACS Nano 2013, 7, 8680−8692. (43) Yang, L.; Knowles, K. E.; Gopalan, A.; Hughes, K. E.; James, M. C.; Gamelin, D. R. One-Pot Synthesis of Monodisperse Colloidal Copper-Doped CdSe Nanocrystals Mediated by Ligand−Copper Interactions. Chem. Mater. 2016, 28, 7375−7384. (44) Iwamura, M.; Takeuchi, S.; Tahara, T. Ultrafast Excited-State Dynamics of Copper(I) Complexes. Acc. Chem. Res. 2015, 48, 782− 791. (45) Iwamura, M.; Takeuchi, S.; Tahara, T. Real-Time Observation of the Photoinduced Structural Change of Bis(2,9-dimethyl-1,10phenanthroline)copper(I) by Femtosecond Fluorescence Spectroscopy: A Realistic Potential Curve of the Jahn−Teller Distortion. J. Am. Chem. Soc. 2007, 129, 5248−5256. (46) Hoffeditz, W. L.; Katz, M. J.; Deria, P.; Cutsail Iii, G. E.; Pellin, M. J.; Farha, O. K.; Hupp, J. T. One Electron Changes Everything. A Multispecies Copper Redox Shuttle for Dye-Sensitized Solar Cells. J. Phys. Chem. C 2016, 120, 3731−3740. (47) Sanz, L.; Palma, J.; García-Quismondo, E.; Anderson, M. The Effect of Chloride Ion Complexation on Reversibility and Redox Potential of the Cu(II)/Cu(I) Couple for Use in Redox Flow Batteries. J. Power Sources 2013, 224, 278−284. (48) Bard, A.; Faulkner, L. Electrochemical Methods: Fundamentals and Applications, second ed.; John Wiley & Sons: New York, 2000. (49) Yang, C.; Faust, A.; Amit, Y.; Gdor, I.; Banin, U.; Ruhman, S. Impurity Sub-Band in Heavily Cu-Doped InAs Nanocrystal Quantum Dots Detected by Ultrafast Transient Absorption. J. Phys. Chem. A 2016, 120, 3088−3097. (50) Sewall, S.; Cooney, R.; Anderson, K.; Dias, E.; Kambhampati, P. State-to-State Exciton Dynamics in Semiconductor Quantum Dots. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 235328. (51) Klimov, V. I. Optical Nonlinearities and Ultrafast Carrier Dynamics in Semiconductor Nanocrystals. J. Phys. Chem. B 2000, 104, 6112−6123.

(52) Hendry, E.; Koeberg, M.; Wang, F.; Zhang, H.; de Mello Donegá, C.; Vanmaekelbergh, D.; Bonn, M. Direct Observation of Electron-to-Hole Energy Transfer in CdSe Quantum Dots. Phys. Rev. Lett. 2006, 96, 057408. (53) Pandey, A.; Guyot-Sionnest, P. Slow Electron Cooling in Colloidal Quantum Dots. Science 2008, 322, 929−932. (54) Efros, A. L.; Kharchenko, V. A.; Rosen, M. Breaking the Phonon Bottleneck in Nanometer Quantum Dots: Role of Auger-like Processes. Solid State Commun. 1995, 93, 281−284. (55) Klimov, V. I.; McBranch, D. W.; Leatherdale, C. A.; Bawendi, M. G. Electron and Hole Relaxation Pathways in Semiconductor Quantum Dots. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 60, 13740−13749. (56) Knowles, K. E.; Tice, D. B.; McArthur, E. A.; Solomon, G. C.; Weiss, E. A. Chemical Control of the Photoluminescence of CdSe Quantum Dot−Organic Complexes with a Series of Para-Substituted Aniline Ligands. J. Am. Chem. Soc. 2010, 132, 1041−1050. (57) Debnath, T.; Sebastian, D.; Maiti, S.; Ghosh, H. N. Tuning Hole and Electron Transfer from Photoexcited CdSe Quantum Dots to Phenol Derivatives: Effect of Electron-Donating and -Withdrawing Moieties. Chem. - Eur. J. 2017, 23, 7306−7314. (58) Sarkar, S.; Guria, A. K.; Pradhan, N. Influence of Doping on Semiconductor Nanocrystals Mediated Charge Transfer and Photocatalytic Organic Reaction. Chem. Commun. 2013, 49, 6018−6020. (59) Burda, C.; Link, S.; Mohamed, M.; El-Sayed, M. The Relaxation Pathways of CdSe Nanoparticles Monitored with Femtosecond TimeResolution from the Visible to the IR: Assignment of the Transient Features by Carrier Quenching. J. Phys. Chem. B 2001, 105, 12286− 12292. (60) Rawalekar, S.; Kaniyankandy, S.; Verma, S.; Ghosh, H. N. Ultrafast Charge Carrier Relaxation and Charge Transfer Dynamics of CdTe/CdS Core−Shell Quantum Dots as Studied by Femtosecond Transient Absorption Spectroscopy. J. Phys. Chem. C 2010, 114, 1460−1466. (61) Kaniyankandy, S.; Rawalekar, S.; Verma, S.; Palit, D. K.; Ghosh, H. N. Charge Carrier Dynamics in Thiol Capped CdTe Quantum Dots. Phys. Chem. Chem. Phys. 2010, 12, 4210−4216.

27240


Electrochemical Evaluation of Dopant Energetics and the Modulation

Recommend Documents