Plasmon-Enhanced Energy Transfer in Photosensitive Nanocrystal Device. Hilmi Volkan Demir 1,4 *

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1 Plasmon-Enhanced Energy Transfer in Photosensitive Nanocrystal Device Shahab khavan, 1,2# Mehmet Zafer kgul, 1,3# Pedro Ludwig Hernandez-Martinez, 1,4# and Hilmi Volkan Demir 1,4 * 1 UNM Institute of Materials Science and Nanotechnology, Department of Electrical and Electronics Engineering and Department of Physics, Bilkent University, nkara, 68, Turkey 2 Presently at the Department of Engineering, University of Cambridge, Cambridge, CB3F, United Kingdom 3 Presently at ICFO Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Castelldefels, Barcelona, 886, Spain 4 LUMINOUS! Center of Excellence, School of Electrical and Electronic Engineering and School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, , Singapore *Corresponding uthor, volkan@stanfordalumni.org, hvdemir@ntu.edu.sg 1

2 S1. The diagram of the device fabrication. 1 Deposition of HfO2 via LD on ITO coated glass 2 Coating of four bilayers of polyelectrolyte bilayers 3 Coating of single layer of donor NCs 4 Coating of three bilayers of polyelectrolyte bilayers 5 Coating of single layer of u MNPs 6 Coating of seven bilayers of polyelectrolyte bilayers 7 8 Coating of single layer of acceptor NCs Evaporation of l contact Figure S1. The diagram of device fabrication for trilayer structure. Device fabrication: To fabricate photosensitive nanocrystal monolayer (PNM), ITOcoated glass substrate was cleaned properly. Then, to create extra hydroxyl group, substrates were exposed to oxygen plasma. The hydroxyl groups are essential for the atomic layer deposition (LD) of high dielectric constant layer such as HfO2 in this case. We deposited 5 nm HfO2 via LD at 2 C. LD is preferred to other deposition systems as it provides a uniform and controlled surface coverage of dielectric film in 2

3 addition to preferentially coats hydrophilic surfaces. In LD, because of the purging water pulses, the quality of the deposited film is improved. fter that, oppositely-charged polyelectrolyte bilayers, nanocrystals (NCs), and u metal nanoparticles (MNPs) were coated via a fully automatic computer controlled dip-coater. Finally, l contact layer (with the thickness of 75 nm) was coated immediately via thermal evaporator. Device characterization: The photovoltage build-up characteristics of PNMs were recorded under Xenon light source. Monochromator was integrated to the Xenon light and gilent Technologies B15 semiconductor parameter analyser was used to characterize the device. The light intensity at each wavelength was counted via Newport optical powermeter. During devices characterization, there was no external bias across the devices and they were connected to shunt resistors (1 MΩ). In order to prevent from mixing he second harmonic, for wavelengths more than 5 nm, FSQ-GG 4 Newport (with a cut-off 4 nm) was used. Layer-by-layer assembly: To coat NCs and u MNPs via dip-coating, a fully automatic computer controlled dip-coater was used. single monolayer of CdSe/CdZnS NCs (negatively charged) was coated on top of pairs of Poly(diallyldimethylammonium chloride) called as PDD (strongly positive) and poly(sodium 4-styrenesulfonate) called as PSS (strongly negative) polyelectrolyte polymers, deposited on 5 nm-thickness HfO 2 layer, and followed by a final layer of PDD (positively charged). Since NaCl improves the coating quality by improving the polymer chain relaxation on the surface, the concentrations of these polyelectrolytes were adjusted to 2 mg/ml in.1 M NaCl solution. Standard cyclic procedure was used during self-assembly procedure: first, substrate was dipped for 5 min into the PDD solution, then it was washed for 1 min 3

4 with water, subsequently dipped for 5 min in PSS solution, and then washed with water for 1 min. fter, obtaining desired number of PDD/PSS pairs, one extra layer of PPD was coated and rinsed with water. Then the sample was dipped for 2 min into the NCs solution and rinsed for 1 min again with water. 4

5 S2. Transmission electron microscopy images of the donor NCs, acceptor NCs and u MNPs. (a) (b) (c) Figure S2. Transmission electron microscopy images of (a) donor CdSe/CdZnS NCs and (b) acceptor CdSe/CdZnS NCs after ligand exchange process using negatively charged mercaptopropionic acid. (c) Transmission electron microscopy image of negatively charged capped thioglycolic acid u MNPs. 5

6 S3. Time resolved fluorescence measurements of CdSe/CdZnS NCs in solution. Photoluminescence Intensity (counts) cceptor CdSe/CdZnS NCs (τ avg = ns) Donor CdSe/CdZnS NCs (τ avg = ns) Time (ns) Figure S3. Time resolved fluorescence measurements of CdSe/CdZnS NCs for donor and acceptor NCs in solution at 53 nm and 661 nm, respectively. 6

7 S4. Theoretical Model We present a theoretical approach to the problem of plasmon coupled nonradiative energy transfer in semiconductor NCs. The goal is to describe the optical properties of MNPs and semiconductor NCs. The main resulting interactions are exciton-exciton and exciton-plasmon interactions. We separate our problem into three cases: 1) Förster-type nonradiative energy transfer (FRET) in donor-acceptor NCs; 2) Plasmon enhanced (donor/acceptor) NCs; and 3) Plasmon coupled FRET donor-u MNPs-acceptor. FRET in NCs pairs We proceed to estimate the FRET rate for donor-acceptor (D-) NC pair. Under the steady-state condition, the number of exciton in donor (acceptor) the rate equations N N is given by D ( ) = (1) D, r D, nr FRET N D ID, abs = (2), r, nr N FRET N D I, abs where is the radiative rate of the donor (acceptor), D, r, r nonradiative rate of the donor (acceptor), and ( I ) I D abs, abs is the D, nr, nr, is the intensity of light absorption in the donor (acceptor). The D- interaction is Förster-type given by: 7

8 1) Single donor NC to 2D acceptor NC π FRET = D σ 2 R d 6 4 (3) Here, R is the Förster radius 1 ; D = D, r + D, nr is the donor s exciton recombination rate; σ is the acceptor concentration; and d is the D- center-to-center separation distance. Then, the emission intensity for the donor (acceptor) is I = N F = I ( ω) ( ω ω ) D D, emiss D, r D D, D, emiss D+ FRET (4) I ( ω) N F( ω ω ) I I FRET, D, abs, emiss =, r = 1+,, emiss D+ FRET I,, abs (5) ( ω ω ) I ω = Y I F is the donor(acceptor) emission where, D, emiss D, D, abs D 2 ω intensity; = a exp F is a Gaussian function for the emission intensity ω 2σ distribution and a is a constant which depends on the unit system. = Y D, r D D and ( 1 Y ) = are the donor(acceptor) radiative and nonradiative exciton D, nr D D recombination rate; and Y D is the donor(acceptor) quantum yield. The respective emission enhancement factors are 8

9 η D, FRET ( ω) I D, emiss D = = I ω +, D, emiss D FRET (6) η, FRET ( ω) ( ω) I, emiss = = 1+ I +,, emiss FRET (7) Plasmon enhanced NCs Here, we consider NCs interacting with MNPs in the presence of a constant electric field. The number of excitons ( N NC) in the NC, under steady-state condition, is given by ( ) + + N + I = (8) NC, r NC, nr nr, metal NC NC, abs where I NC, abs is the intensity of light absorption in the NC, NC, r ( NC, nr) is the NC radiative (nonradiative) rate, and nr,metal is calculated by 2,3 nr, metal is the energy transfer rate from the NC to the MNP. 1) Single NC to 2D MNPs ed exc π R MNP 3ε nr, metal, α = bα MNP Im 4 MNP h ε eff 2 d 2ε + ε MNP( ω) σ ε ω (9) 9

10 1 4 where b 1 α =., for α = x, y, z, ε is the outside medium dielectric constant, ( ω) ε MNP, σ MNP, R MNP are the dielectric function, the concentration, and radius of the MNP, respectively, d exc is the NC s exciton dipole moment, d is the NC-MNP center-to-center separation distance, and ε eff is the effective dielectric constant: ε eff 2ε + ε 3 NC = (1) t room temperature, the total energy transfer rate ( nr, metal) from the NC to the MNP is nr, metal x+ y+ z = (11) 3 We assume that NC, nr =, NC, nr does not change in the presence of MNP while the radiative rate and absorption intensity are modified by the MNP as 4 = ω (12) NC, r LSP, NC, r ( ω ), I = I (13) abs LSP abs,,, where, NC, r NC nr, and I,abs are the parameters in the absence of MNP; ω LSP is the u MNP plasmon frequency. The electric field enhancement factor ( ω) is defined as 1

11 ( ω) = NC NC E in, NC E 2 2 dv dv (14) E E is the electric field in the NCs in the presence (absence) of MNP. The where, in, NC emission intensity is written as ( ω ) I I N F F NC, r LSP, abs emiss ω = NC, r NC ω ωexc = ω ωnc NC + nr, metal (15) = + is the NC exciton recombination rate in the presence of metal NC NC, r NC, nr nanoparticle, and ( LSP ) factor ω NC is the NC emission frequency. The NC emission enhancement η ω in the presence of MNP is η LSP ( ω ) laser ( ω) ( ) Iemiss ωlsp NC = = I ω 1 + ω 1 Y +, emiss LSP NC NC nr, metal (16) Plasmon coupled FRET donor-u MNPs-acceptor In this section, we calculate plasmon coupled FRET for the case of D- NC in the presence of MNP and estimate the emission intensity for the donor and the acceptor NC. The rate equations for the donor and acceptor under the steady-state condition are 11

12 ( ) = (17) D, r D, nr D, nr, metal LSP FRET ND I D, abs = (18), r, nr, nr, metal N LSP FRET ND I, abs where N N is the number of exciton in the donor (acceptor), D = ( ω, LSP) D r, D ;, r D, nr, D, nr = ; I ( ω, LSP) I,, = are the D abs D D abs radiative rate, nonradiative rate, and absorption intensity of the donor(acceptor) in the presence of MNP, respectively. D ω is the donor(acceptor) NC electric field LSP enhancement factor in the presence of MNP. ( is the donor(acceptor) NC D ), nr, metal exciton transfer rate to the MNP given by: 1) Single donor(acceptor) NC to 2D MNP ed D, exc π RMNP 3ε D = b, nr, metal, α α 4 h ε ε + ε ω D eff σ MNP Imε MNP ω 2 d, 2 D MNP (19) 1 4 where b 1 α =., for α = x, y, z respectively, σ MNP is the MNP concentration, ε is the outside medium dielectric constant, ε MNP ( ω) is the MNP dielectric function, R MNP is the MNP radius, d D( is the donor(acceptor) NC exciton dipole moment, d ), exc D is the NC- MNP center-to-center separation distance, and ε D( is the donor(acceptor) effective ), eff dielectric constant 12

13 ε, D eff 2ε + ε D = (2) 3 t room temperature, the total energy transfer rate ( nr, metal) from the NC to the MNP is nr, metal x+ y+ z = (21) 3 The enhance FRET is calculated as follow: 1) Single donor NC to 2D acceptor NC π LSP FRET = D σ 2 R 6, LSP 4 d (22) Here, we assumed that the Förster radius is enhanced by: ( ω ) R = R (23), LSP D LSP Thus, the donor (acceptor) emission intensity, I ( ω) = N F( ω ω ) ( I emiss( ω) r N F( ω ω) ) = is,,, D, emiss D, r D D 13

14 = ( ) D LSP D D, emiss ω D, D, emiss ω ωd D+ D, nr, metal + LSP FRET I Y I F ω (24) ( ω ) ( ω ) 1 LSP LSP FRET D LSP,D, emiss, emiss ω =,, emiss ω ω + +, nr, metal D+ D, nr, metal + LSP FRET ω LSP I,, emiss I Y I F I (25) Thus, the donor(acceptor) emission enhancement factor is given by η D, LSP FRET D ωlsp D = 1 + ( ( ω ) 1) Y + + D LSP D D D, nr, metal LSP FRET (26) η, LSP FRET ( ( ω ) ) Y ωlsp = 1+ LSP 1 +, nr, metal LSP FRET 1+ 1 ( ( ω ) 1) Y D LSP D D D, nr, metal LSP FRET ( ω ) ( ω ) I D LSP,D, emiss I LSP,, emiss (27) 14

15 Numerical results Here, we present our numerical results for FRET for the three cases mentioned above. The parameters we used are: R = 2.5 nm, R = 2.4 nm, R = 3.1 nm, MNP DQD QD λ = 55 nm, λ = 664 nm, D 13 σ D = particles/cm 2, 13 σ = 1 1 particles/cm 2, 11 σ MNP = 1 1 particles/cm 2,, D.3 Y =, Y, =.5, τ, = 1.8 ns, τ, = 5.43 ns, D τ = 5.44 ns, τ = 4.72 ns. We consider the dielectric constant for the outside media as D ε m ε PDD+ ε PSS = (28) 2 where ε = 2. is the dielectric constant of the PDD and ε = is the dielectric PDD constant of the PSS. lso, we consider that the thickness of the PDD/PSS layer to be 1.2 nm. Figure S4 shows the photoluminescence (PL) intensity of our system. Wine dash line represents donor PL intensity without coupling. Orange dot line represents acceptor PL intensity without coupling. Green dash-dot line illustrates the PL intensity of the donor when it is coupled to the u MNPs. Red dash-dot-dot line shows the PL intensity of the acceptor when is coupled to the u MNPs. Pink small dash line illustrates the FRET. Blue solid line shows the FRET for the D- NC pair when NCs are coupled to the u MNP. From this figure, it can be observed that when the donor is coupled to u MNPs, its PL quenches 47%. On the other hand, the acceptor increases its PL intensity by 15%. In the case of D- FRET, the donor quenches its PL by 11% while the acceptor enhances its PL by 1%. Finally, in the case of plasmon enhance FRET, the donor decreases its PL PSS 15

16 by 65%, while the acceptor increases its PL by 63%. PL Intensity (a.u) Donor cceptor u MNPs - Donor u MNPs - cceptor Donor - cceptor Donor - u MNPs - cceptor Wavelength (nm) Figure S4: Photoluminescence intensity for the Donor-cceptor NC pair when MNP is coupled to the NCs. Wine dash line represents donor PL intensity without coupling. Orange dot line represents acceptor photoluminescence intensity without coupling. Green dash-dot line illustrates the PL intensity of the donor when it is coupled to the u MNPs. Red dash-dot-dot line shows the PL intensity of the acceptor when is coupled to the u MNPs. Magenta small dash line illustrates the FRET. Blue solid line shows the FRET for the D- NC pair when NCs are coupled to the u MNP. 16

17 S5. Variation in the charge accumulation over time. Figure S5, shows variation in the voltage buildup while the device is under illumination for a long time. When the light is on, there is a net positive potential buildup (1). fter it get to the peak point (device is still under illumination), voltage buildup starts to decrease until it reaches the steady state (2). fter switching off the light, the net voltage reaches the lowest negative level (3), this is the effect of trapped charges. Then, net potential gets back to its initial value (zero level) (4) and this recovery duration indicates the time required to get rid of the trapped charges. 1 Voltage Buildup (mv) 5 (1) (2) (3) (4) Light is on Light is off Time (s) Figure S5. Changes in the voltage buildup as a function of time by turning the incident light on and off (.73 mw/cm 2 at 35 nm). 17

18 S6. Photovoltage buildup comparison between the single donor NC layer and the u MNPs-donor devices. The voltage buildup gets larger with reducing the excitation wavelengths. Since NCs have higher density of states at lower wavelengths, resulting in more electrons and holes are photogenerated at higher photon energies. Likewise, the negative voltage value at a longer excitation wavelength is lower since fewer number of charges are trapped inside the NCs. In the case of u MNPs-donor where the separation distance between the MNPs layer and NCs layer is too short, the positive voltage buildup is decreased due to increased nonradiative processes by u MNPs. Similarly, less number of charges are trapped inside the NCs in the u MNPs-donor device (Table S6 and Figure S6a). Figure S6b presents the sensitivity comparison between the donor and the u MNPs-donor based devices as a function of the excitation wavelength. 18

19 Excitation λ Donor u MNPs-Donor (nm) Positive photovoltage Negative voltage (mv) Positive photovoltage Negative voltage(mv) buildup (mv) buildup (mv) Table S6. Photovoltage buildup values for single donor NC layer and u MNPs-donor PNM devices at different excitation wavelengths. In the PNM device, positive photovoltage buildup come from positive charges (holes) that accumulation in the top l electrode. Negative voltage value is due to electrons that tend to stay in the NCs and get trapped inside the NCs as deep trap states; these are the long-lived trap states. 19

20 Voltage Buildup (mv) (a) Donor u MNPs-Donor Photosensitivity (V/W) (b) Donor Donor-u MNPs Wavelength (nm) Wavelength (nm) Figure S6. (a) Changes in the photovoltage buildup at various wavelengths for single donor NCs layer and u MNP-donor PNMs. (b) Sensitivity curve for single donor NCs layer and u MNP-donor PNMs. 2

21 S7. Photovoltage buildup comparison between the single acceptor NC layer and the u MNPs-acceptor devices. comparison of photovoltage buildup at different excitation wavelengths for the acceptor and the u MNPs-acceptor PNMs. When there is sufficient inter space between MNPs and NCs layer, the local electric field enhanced by u MNPs affect the photogeneration kinetics of NCs. The increment in the number of excitons can be deduced from the enhancement in positive photovoltage buildup and negative voltage, see Table S7 and Figure S7a. Here, the sensitivity was improved over the range of 35 nm to 575 nm. The maximum level of enhancement is 2.38-fold at around 525 nm (u MNPs LSP resonance peak), compared to the control sample (Figure S7b). Excitation λ cceptor u MNPs-cceptor (nm) Positive photovoltage Negative voltage (mv) Positive photovoltage Negative voltage (mv) buildup (mv) buildup (mv) Table S7. Photovoltage buildup values for single acceptor NC layer and u MNPsacceptor PNM devices at different excitation wavelengths. 21

22 Voltage Buildup (mv) (a) cceptor u MNPs-cceptor Photosensitivity (V/W) (b) cceptor u MNPs-cceptor Wavelength (nm) Wavelength (nm) Figure S7. (a) Changes in the photovoltage buildup at various wavelengths for single acceptor NCs layer and u MNPs-acceptor PNMs. (b) Sensitivity curve for single acceptor NCs layer and u MNPs-acceptor PNMs. 22

23 S8. Photovoltage buildup comparison between the single acceptor NC layer and the donor-acceptor bilayer devices. Photovoltage buildup of PNMs at different excitation wavelengths for single acceptor NC layer and donor-acceptor bilayer. The donor-acceptor bilayer device shows slight enhancement in voltage buildup relative to the single acceptor NC layer, which is a consequence of the FRET between donor and acceptor NCs. However, enhancement is inferior due to the excessively large surface to surface separation ( 22 nm) between the NCs. Similarly, relative to the trapped charges in single acceptor layer, larger number of electron are trapped inside the acceptor NCs in donor-acceptor bilayer, see Figure S8a and table S8. However, the difference is not as significant as in u MNPs-acceptor device. In addition, photosensitivity of the devices at different excitation wavelength and different intensities are presented in Figure S8b. 23

24 Excitation λ cceptor Donor-cceptor (nm) Positive photovoltage Negative voltage (mv) Positive photovoltage Negative voltage (mv) buildup (mv) buildup (mv) Table S8. Photovoltage buildup values for single acceptor NC layer and donor-acceptor PNM devices at different excitation wavelengths. Voltage Buildup (mv) (a) cceptor Donor-cceptor Photosensitivity (V/W) (b) cceptor Donor-cceptor Wavelength (nm) Wavelength (nm) Figure S8. (a) Changes in the photovoltage buildup at various wavelengths for single acceptor NCs layer and donor-acceptor bilayer PNMs. (b) Sensitivity curve for single acceptor NCs layer and donor-acceptor bilayer PNMs. 24

25 S9. Photovoltage buildup comparison between the single acceptor NC layer and the donor-u MNPs-acceptor (trilayer) devices. comparison of photovoltage buildup at different excitation wavelengths for the single acceptor layer and the trilayer (donor-u MNPs-acceptor) PNMs. s can be seen in the table S9 and Figure S9a, the photovoltage buildup and the number of the trapped charges are increased compared to the single acceptor layer. Moreover, compared to the u MNPs-acceptor and donor-acceptor PNMs, there is a significant enhancement in both photovoltage buildup and the number of the trapped charges. Corresponding photosensitivity curve of the devices as a function of excitation wavelengths is given in Figure S9b. Excitation λ cceptor Donor-u MNPs-cceptor (nm) Positive photovoltage Negative voltage (mv) Positive photovoltage Negative voltage (mv) buildup (mv) buildup (mv) Table S9. Photovoltage buildup values for single acceptor NC layer and donor-u MNPs-acceptor trilayer PNM devices at different excitation wavelengths. 25

26 Voltage Buildup (mv) (a) cceptor Donor-u MNPs-cceptor Photosensitivity (V/W) (b) cceptor Donor-u MNP-cceptor Wavelength (nm) Wavelength (nm) Figure S9. (a) Changes in the photovoltage buildup at different excitation wavelengths for single acceptor NCs layer and donor-u MNPs-acceptor trilayer PNMs. (b) Sensitivity curve for single acceptor NCs layer and donor-u MNPs-acceptor trilayer PNMs. 26

27 S1. Photovoltage buildup comparison between the single donor NC layer, the u MNPs-donor, and the acceptor-u MNPs-donor (reverse trilayer) devices. Photovoltage buildup of PNMs at different excitation wavelengths and intensities for single donor NC layer, u MNPs-donor, and the reverse trilayer structure (acceptor-u MNPs-donor). The photovoltage buildup and trapped charges are significantly reduced compared to the single donor layer structure, because of the migration of excitons toward the acceptor layer while donor NC layer is placed underneath the l electrode. Likewise, relative to the u MNPs-donor where short distance between MNPs layer and NC layer increased nonradiative channels, there is still a reduction for reverse structure in the photovoltage buildup and trapped charges (Table S1 and Figure S1a). This indicates the importance of LSP-coupled FRET where the exciton transfer is assisted significantly and migrated away from the l electrode. Subsequently, less number of electrons are trapped inside the donor NC layer and few holes get accumulate at l contact. Eventually, all these parameters result in less photosensitivity as can be seen in Figure S1b. 27

28 Excitation λ Donor u MNPs-Donor cceptor-u MNPs-Donor (nm) Positive Negative voltage Positive Negative voltage Positive Negative voltage photovoltage (mv) photovoltage (mv) photovoltage (mv) Table S1. Photovoltage buildup values for single donor NCs layer, u MNPs-donor, and acceptor-u MNPs-donor reverse trilayer PNM devices at different excitation wavelengths. Voltage Buildup (mv) (a) Donor u MNPs-Donor cceptor-u MNPs-Donor Photosensitivity (V/W) (b) Donor u MNPs-Donor cceptor-u MNPs-Donor Wavelength (nm) Wavelength (nm) Figure S1. (a) Changes in the photovoltage buildup at various wavelengths for single donor NCs layer, u MNPs-donor, and acceptor-u MNPs-donor reverse trilayer PNMs. (b) Sensitivity curve for single donor NCs layer, u MNPs-donor, and acceptor-u MNPs-donor reverse trilayer PNMs. 28

29 References 1. Lakowicz, J. R.Principles of Fluorescence Spectroscopy; Springer, Govorov,. O.; Lee, J.; Kotov, N.. Theory of Plasmon-Enhanced Förster Energy Transfer in Optically Excited Semiconductor and Metal Nanoparticles. Phys Rev B 27 76, Hernández-Martínez, P. L.; Govorov,. O.; Demir, H. V. Förster-type Nonradiative Energy Transfer for ssemblies of rrayed Nanostructures: Confinement Dimension vs Stacking Dimension. J. Phys. Chem. C , Govorov,. O.;Bryant, G. W.;Zhang, W.; Skeini, T.; Lee, J.; Kotov, N..; Slocik, J. M.;Naik, R. R. Exciton-Plasmon Interaction and Hybrid Excitons in Semiconductor-Metal Nanoparticle ssemblies. Nano Letter 26 6,