Semiconductor Nanostructures with Efficient Energy

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Hybrid Dendrimer/Semiconductor Nanostructures with Efficient Energy Transfer via Optical Waveguiding Tzu-Neng Lin, Jiun-Chun Huang, Ji-Lin Shen, Chien-Ming Chu, JuiMing Yeh, Yui Whei Chen-Yang, Ching-Hsueh Chiu, and Hao-Chung Kuo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5111949 • Publication Date (Web): 09 Feb 2015 Downloaded from http://pubs.acs.org on February 11, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Hybrid Dendrimer/Semiconductor Nanostructures with Efficient Energy Transfer via Optical Waveguiding T. N. Lin1, J. C. Huang1, J. L. Shen1*, C. M. Chu2, J. M. Yeh2, Y. H. Chen-Yang2, C. H. Chiu3, and H. C. Kuo4

1

Department of Physics and Center for Nanotechnology, Chung Yuan Christian University, Chung-Li, Taiwan

2

Department of Chemistry and Center for Nanotechnology, Chung Yuan Christian University, Chung-Li, Taiwan

3

Advanced Optoelectronic Technology Inc., Hsinchu, Taiwan

4

Department of Photonic and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsin-Chu, Taiwan

*Corresponding author:

Tel.:+886-3-2653227 ; FAX: +886-3-2653299 E-mail: [email protected] (Ji-Lin Shen)

1

ACS Paragon Plus Environment


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ABSTRACT

In this study we demonstrated a nonradiative energy transfer between InGaN quantum wells (QWs) and polyamidoamine (PAMAM) dendrimers through optical waveguiding by detecting a reduction in the photoluminescence lifetime. The maximum energy transfer efficiency of this system is ~72%, which is promising for sensor applications. The energy transfer efficiency depends on the distance between the InGaN QWs and PAMAM dendrimers by a factor of 1/d2, revealing layer-to-layer dipole interactions. Upon increasing the generation of PAMAM dendrimers, the transfer energy efficiency increases exponentially, which can be explained by the increased surface coverage for the higher generation.

Keywords: PAMAM; InGaN; quantum well; fluorescence resonance energy transfer; generation effect

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1. Introduction Dendrimers are hyperbranched molecules with nanometer-scale dimensions which can be synthesized through an iterative sequence of reactions.1-2 The structure of dendrimers is composed of a central core and symmetric branches, radiating from the central core. The superior characteristics of dendrimers include numerous terminal groups, controllable molecular weight, size, shape, and surface reactivity. A great deal of attention has been paid to this class of macromolecules owing to its potential applications in optoelectronic devices, artificial light-harvesting systems, analytical chemistry and biomedicine.1-4 Advanced progress of functional dendrimers in redox-dendrimer engineering, pharmaceutical development, and solar energy conversion has recently been reviewed.5-7 Dendrimers are ideal scaffolds to decorate with a lot of chromophores on the surface, resulting in an efficient capture of photons or generating fluorescence. For example, the encapsulated dye molecule accumulates energy from many chromophoric units of dendrimers very efficiently.8 A system built by dendrimers, NdIII ions, and RuII complexes can then behave as an antenna which harvests ultraviolet to visible light.9 Furthermore, dendrimers with well-defined pseudorotaxane assemblies lead to a striking enhancement in the quantum yield of fluorescence dendrimers.10 Dendrimers can thus be used as light-harvesting antennae or fluorescence markers in environmental applications.11-12 Among a variety of

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dendrimers, poly(amidoamine) (PAMAM) dendrimers are a well-known class of nanoscopic, spherical, and monodistributed molecules having hydroxyl or amino groups on the surface. By changing the central core and abundant surface groups, many PAMAM conjugates can be generated which exhibit special properties. These PAMAM conjugates are bio-compatible and suitable for carrying biomaterials, with potential for drug delivery and sensing applications.2, 13-15

Over the past decades, fluorescence resonance energy transfer (FRET), in which donors transfer energy through nonradiative couplings to acceptors, has been widely studied.16-17 FRET takes place when there is a considerable overlap between the absorption spectrum of the acceptors and the fluorescence spectrum of the donors. The FRET efficiency greatly relies on the separation distance between the donors and acceptors. Recently, dendrimers capable of transferring energy over nanometers of distance have garnered increasing interest.15,18-19 Owing to their proximity, the different functional groups of dendrimers can easily couple to the dendrimer branches or the molecules hosted in dendritic cores.12-15,20-22 FRET processes associated with PAMAM dendrimers have been investigated in several studies. For example, PAMAM dendrimers labeled with yellow-green 1,8-naphthalimide dyes were shown to efficiently collect light and transfer energy to Rhodamine 6G dyes.12 A surface energy transfer process was also demonstrated between the mannose-functionalized

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PAMAM dendrimers and gold nanoparticles, and used as a vehicle for proteins detection.22

FRET can also be attained between inorganic semiconductor quantum wells (QWs) and organic materials.23-25 The hybrid organic-inorganic FRET system capitalizes on the direct coupling from QWs to acceptor materials in a large area, and provides a noncontact pumping route for electrical carrier injection.24 Thus, FRET in hybrid organic-inorganic structures, can provide an alternate pumping scheme to implement electroluminescence in dendrimer-based light emitting devices. In this study, we propose a new dendrimer-based FRET system, where the energy from the InGaN QWs can be transferred to PAMAM dendrimers via optical waveguiding. Fig. 1 displays the schematic representation of the energy transfer system. The steady-state and time-resolved photoluminescence (PL) were used to demonstrate the energy transfer from InGaN QWs to PAMAM dendrimers. The mechanism of the energy transfer process was examined by the distance-dependent interactions between InGaN QWs and PAMAM dendrimers. The generation effect on the energy transfer efficiency from the InGaN QWs to the PAMAM dendrimers was also investigated.

2. Experimental Methods In the present study, a single InGaN/GaN QW, grown by metal-organic chemical vapor deposition (MOCVD) on sapphire (0001) substrates, was used as the 5



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donor.26 A 2-nm-thick single QW was grown after growing a GaN buffer layer with a thickness of 2.00±0.01 µm. To control the separation distance between donors and acceptors, four different thicknesses of GaN cap layers (2, 4, 6, and 8 nm) were grown on the single QW. The acceptor (G0, G1, G2, and G3) PAMAM dendrimers were synthesized using a divergent method by repeating the Michael addition of the amino group with methyl acrylate monomers. The subsequent amidation of the ester groups with excess ethylenediamine resulted in PAMAM dendrimers for the different generations. To perform the energy transfer experiment, a drop-cast method was used to introduce PAMAM dendrimers on the top of the QW sample. The excitation source for the PL measurements was a pulsed laser, operating at a wavelength of 260 nm, a duration of 250 fs, and a repetition frequency of 20 MHz. The collected time-integrated PL was detected with a high-speed photomultiplier tube. The technique of time-correlated single-photon counting (TCSPC) was used for detecting the time-resolved PL. The time resolution of the instrument response in our system was around 200 ps. The PL measurements were carried out at room temperature.

3. Results and Discussion The PL spectrum of the InGaN QW (donor) with a 2-nm-thick cap layer is displayed as the solid line in Fig. 2. A narrow PL peak at around 404 nm can be clearly observed. The PL spectrum of the G3 PAMAM dendrimer is displayed in the

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open squares in Fig. 2, revealing a broad band located at ~465 nm. The PL properties of PAMAM dendrimers have been investigated previously and a similar PL band was also observed.27-29 It was found that the backbone of the PAMAM dendrimers was insignificant for generating PL. On the other hand, the terminal groups on the dendrimer surface were responsible for the PL in PAMAM dendrimers.27 The PL of PAMAM dendrimers is also greatly influenced by pH values, temperature, aging time, concentration, and oxidation.28-29 The PL excitation spectrum of the G3 PAMAM dendrimer is shown in the open circles in Fig. 2, revealing an absorption band with the peak around ~350 nm. The spectral overlap between the InGaN QWs and the G3 PAMAM dendrimers indicates the feasibility of coupling and FRET between them.

The PL spectra of the InGaN QW with a 2-nm cap thickness with (dashed line) and without (solid line) the introduction of G3 PAMAM dendrimers are displayed in Fig. 3. With the introduction of PAMAM dendrimers, a clear decrease in the PL intensity of the QW and a simultaneous appearance in the PL of PAMAM dendrimers were observed. The PL decay of the G3 PAMAM dendrimer with (open triangles) and without (open circles) the introduction of the InGaN QW is displayed in the inset of Fig. 3. Obviously, the PL decay time of G3 PAMAM dendrimers with the InGaN QW increased considerably. The decrease in the PL intensity in InGaN QWs (donor), as well as the increase in the intensity and PL decay time in the G3 PAMAM dendrimer

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(acceptor), provides strong evidences that FRET occurred between them. Fig. 4 shows a photograph of the hybrid G3 PAMAM/QW sample upon optical excitation. A circular light-blue image with an area of 0.35±0.01 cm2 was observed in the area where the PAMAM dendrimers were placed. It is notable that the image in Fig. 4 was detected without the use of a notch filter, which is used to reduce the light from the laser source. The light from the PAMAM dendrimer can be observed by the naked eye. These observations served to confirm the energy transfer between the InGaN QW and the G3 PAMAM dendrimer. The dendrimer-based energy transfer with optical waveguiding has advantages for large-area and multiple-acceptor detection for on-chip device applications.

The time-resolved PL of the bare QW and the hybrid PAMAM/QW was performed under identical conditions to further investigate the energy transfer between QWs and PAMAM dendrimers. By characterizing the time-resolved PL, the FRET between donors and acceptors could be identified. A decrease (increase) in the PL decay time of donors (acceptors) could be detected as energy was transferred from donors to acceptors nonradiatively; the PL decay time in donors (acceptors) remained unaltered if no FRET occurred. PL decay profiles of InGaN QWs with (open triangles) and without (open circles) G3 PAMAM dendrimers for four samples with various thicknesses of cap layers are shown in Fig. 5. The PL decay results for all samples

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revealed that the PL decay time in the QWs was reduced after the introduction of the PAMAM dendrimer. This confirmed the existence of an extra decay path for the energy transfer from the InGaN QW to the PAMAM dendrimer. Owing to the presence of phase separation and/or spatial disorder, the PL decay in the InGaN QW can be described by a distribution of the PL decay time. A stretched-exponential function was thus used to fit the PL decay profiles in Fig. 5:26,30 β

n ( t ) = n ( 0 ) e − ( k t) ,

(1)

where n (t ) , k, and β represent the donor density, the carrier decay rate in donors and a dispersive factor, respectively. The fitted curves, using Eq. (1), displayed as solid lines in Fig. 5, were in good agreement with the measured data. Table 1 exhibits the corresponding parameters for the fits. The average decay time can be calculated in such a stretched exponential function as follows:31 < τ >=

1 1 Γ( ) , kβ β

(2)

where Γ represents the Gamma function. With the increasing thickness of the cap layer, the decrease in the average decay time was less pronounced after introducing the PAMAM dendrimer. This indicated that the effect of the energy transfer was reduced as the thickness of the cap layer increased.

For the bare InGaN QW, the PL decay time measured from the time-resolved PL

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can be represented by

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τ QW . Owing to an extra energy loss, the PL decay rate of the

hybrid PAMAM/QW sample is represented by:32-33

τ hybrid −1 = τ QW −1 + τ ET −1 ,

(3)

where τ ET is the decay lifetime due to the energy transfer. The corresponding energy transfer rate K ET = τ hybrid

−1

−1

− τ QW , can then be determined from Eq. (3). The ratio

of the photons absorbed by donors that are transmitted to acceptors defines the energy transfer efficiency, Φ ET , which is represented by:25,32-33

Φ ET =

K ET τ QW + K ET

(4)

−1

.

From Eqs. (1)-(3), 〈τ QW 〉 , 〈τ hybrid 〉 , and 〈τ ET 〉 can be obtained and these values allow us to calculate KET and Φ ET . According to Eq. (4) the dependence of Φ ET on the cap thickness was estimated and plotted as circles in Fig. 6. The parameters used for calculating Φ ET are listed in Table 1. As expected, Φ ET reduced monotonically with the increase in thickness of the cap layer. For the QW sample with a 2-nm cap thickness, the maximum efficiency of the energy transfer was found to be as high as 72 ± 6%. The energy transfer efficiency from donors to acceptors can be also obtained from the steady-state PL:26 E = 1−

I DA ID

,

(5)

where IDA and ID represents the PL intensity of donors with and without acceptors, 10


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respectively. On the basis of Eq. (5), the energy transfer efficiency extracted from Fig. 3 was calculated to be 70 ± 5%, which corresponds to the result obtained from the time-resolved measurements (Eq. (4)). The high efficiency of the energy transfer in our system would be an advantage for a more highly sensitive detection for FRET, leading to a potential application in biomedical sensors. The energy transfer rate reveals a dependence of d-6for a traditional FRET, where d is the separation distance between donors and acceptors.32 The non-d-6 dependence

of the energy transfer rate may occur when the interaction between donors and acceptors involves an extended system associated with delocalized charges.33 The dependence of the energy transfer efficiency on the separation distance, d, was analyzed to investigate the energy transfer mechanism in our case. In general, the dependence of the energy transfer efficiency on the separation distance, d, can be represented as:34 Φ ET =

1 , 1 + (d / d 0 ) n

(6)

where d0 represents the distance between donors and acceptors for an energy transfer efficiency of 50%; n represents a parameter which relies on the dipole interaction between the donor and the acceptor; and n = 2, 4, and 6 reveal the interactions between

the

two-dimensional

dipole

to

the

two-dimensional

dipole,

the

two-dimensional dipoles to the point dipole, and the point dipole to the point dipoles 11



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(the traditional FRET), respectively. Thus, n is related to the geometry of the system and can be used to determine the type of energy transfer. Using Eq. (6), the fitted results for n = 2 (solid line), n = 4 (open squares), and n = 6 (open triangles) with d0 = 3.4 nm are as displayed in Fig. 6. As can be seen, n = 4 and n = 6 do not agree with experimental Φ ET as a function of d. However, the fit using n = 2 predicted the experimental results quite well. We, therefore, deduced that the energy transfer from the InGaN QW to the PAMAM dendrimer followed a 1/d2 distance dependence.

It has been found that PAMAM dendrimers are deformed when in contact with a solid surface.35 On the basis of atomic force microscopy experiments and molecular dynamics simulations, the absorbed PAMAM dendrimers have been demonstrated to be no longer spherically symmetric, but resemble flat disks on a substrate or the liquid-solid interface.35-36 In the case of the present study, the carriers in the PAMAM dendrimers were confined in the flattened disk, indicative of two-dimensional behavior. Also, the carriers in the QW were strongly localized in the well plane and characterized as two-dimensional carriers. The energy transfer, as shown in Fig. 6, can thus be best described by an n = 2 dependence, since the interactions between two-dimensional dipoles (squashed PAMAM dendrimers) and two-dimensional dipoles (QWs) are supposed to produce a 1/d2 dependence on the separation distance. In other words, the energy transfer from the InGaN QW to the PAMAM dendrimer

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showed the coupling nature between two two-dimensional layers in this hybrid organic-inorganic system.

The generation effect of PAMAM dendrimers on the transfer energy efficiency of the hybrid organic-inorganic FRET system was also investigated. Fig. 7 displays the PL decay profiles of the QW with a 2-nm cap with (open triangles) and without (open circles) the PAMAM dendrimer for different generations of PAMAM dendrimers. Again, the PL decay results for all samples revealed that the QW decay time decreased after adding the PAMAM dendrimer on top of the QWs. The measured PL decay curves were fitted via Eq. (1); the fitting parameters are collected in Table 2. The fitted results, displayed as the solid lines in Fig. 7, revealed a good agreement with the experimental data. Again, Φ ET versus the generation of PAMAM dendrimers was analyzed according to Eqs. (2)-(5), and is shown as the circles in Fig. 8. The parameters used for calculating Φ ET are listed in Table 2. Φ ET increased monotonically as the generation of PAMAM dendrimers increased. This indicated that higher generation of PAMAM dendrimers exhibited better energy transfer efficiency from the InGaN QW to the PAMAM dendrimer. The maximum energy transfer efficiency calculated from the experiments was 72 ± 6%. The generation-dependent

Φ ET was fitted with an exponential curve, shown as the solid line in Fig. 8. A nice fit between the exponential function and the experimental data indicated that the energy

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transfer in this system increased exponentially with generation. The exponential growth of the energy transfer efficiency can be explained by the increased surface coverage for the higher-generation dendrimers.37 This leads to the increased density in the transition dipole moment and, hence, the increased energy transfer efficiency from the InGaN QWs to the PAMAM dendrimers.

4. Conclusions The distance- and generation- dependence of the energy transfer from InGaN QWs to PAMAM dendrimers via optical waveguiding have been studied. The PL decay time in the InGaN QW decreased noticeably when PAMAM dendrimers were introduced. The energy transfer from QWs to PAMAM dendrimers, described by the 1/d2 separation distance, indicated the layer-to-layer dipole interaction. There was an exponential increase in the transfer energy efficiency as the generation of PAMAM dendrimers increased. The effect of generation on the transfer efficiency was the result of the increased surface coverage of the higher-generation dendrimers.

Acknowledgments

This project was supported in part by the National Science Council under the grant number NSC 102-2632-M-033-001-MY3 and MOST 103-2112-M-033-004-MY3.

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37. Ogasawara, S.; Ikeda, A.; Kikuchi, J. I. Positive Dendritic Effect in DNA/Porphyrin Composite Photocurrent Generators Containing Dendrimers as the Stationary Phase. Chem. Mater. 2006, 18, 5982-5987.

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Figure Captions:

Fig. 1

Schematic representation of the energy transfer system in this study. The dashed arrows represent the energy transfer from an InGaN quantum well (QW) to PAMAM dendrimers.

Fig. 2

Photoluminescence (PL) (open squares) and PL excitation (open circles) spectra of G3 PAMAM dendrimers. The PL spectrum of the InGaN QW is displayed as the solid line.

Fig. 3

PL spectra of the InGaN QW with (dashed line) and without (solid line) of G3 PAMAM dendrimers. The inset shows the PL decays of the G3 PAMAM dendrimer with (open triangles) and without (open circles) the introduction of the InGaN QW.

Fig. 4

Images of the hybrid G3- PAMAM-dendrimer /InGaN-QW sample upon excitation of a UV laser. The left bright spot is the reflection of the excitation laser.

Fig. 5

The PL decay profiles of the InGaN QW with (open circles) and without (open triangles) G3 PAMAM dendrimers for different thicknesses of cap layers: (a) 2 nm, (b) 4 nm, (c) 6 nm, and (d) 8 nm. The solid lines display the fitted curves using Eq. (1).

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Fig. 6

Experimental data (solid circles) and fitted curves of the transfer energy efficiency versus the separation distance. The solid line, open triangles, and open squares display the fitted curves using Eq. (6), with n = 2, 4, and 6, respectively.

Fig. 7

The PL decay profiles of the InGaN QW in the absence (open circles) and presence (open triangles) of PAMAM dendrimers with different generations: (a) G0, (b) G1, (c) G2, and (d) G3. The fitted curves using Eq. (1) are shown as the solid lines.

Fig. 8

The dependence of the measured energy transfer efficiency on the generation of PAMAM dendrimers (solid circles). The solid line shows the fitted curve using an exponential function.

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Table Captions: Table 1

The decay rates k, dispersion components β , the PL decay time of bare QWs , and the PL decay time of QWs in the presence of G3 PAMAM dendrimers fitted from Fig. 5.

Table 2

The decay rates k, dispersion components β , the PL decay time of bare QWs , and the PL decay time of QWs in the presence of PAMAM dendrimers fitted from Fig. 7.

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 6

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Fig. 7

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Fig. 8

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Without PAMAM Cap thickness (nm)

β

K -1

(ns )

(ns)

With PAMAM β

< τ hybrid >

k -1

(ns )

(ns)

2±0.2

0.68

1.79±0.01

0.73±0.01

0.68

5.55±0.25

0.20±0.01

4±0.2

0.68

1.67±0.02

0.78±0.01

0.68

2.86±0.07

0.46±0.02

6±0.2

0.69

1.82±0.04

0.72±0.02

0.70

2.44±0.06

0.53±0.02

8±0.2

0.70

1.22±0.02

1.01±0.02

0.68

1.61±0.04

0.85±0.02

Table 1

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Without PAMAM Generation

β

k

-1

(ns )

(ns)

Page 34 of 35

With PAMAM β

< τ hybrid >

k -1

(ns )

(ns)

0±0.1

0.70

1.11±0.01

1.14±0.01

0.72

1.47±0.05

0.86±0.03

1±0.1

0.70

1.11±0.01

1.14±0.01

0.74

1.56±0.04

0.79±0.02

2±0.1

0.68

0.91±0.01

1.43±0.02

0.73

1.61±0.04

0.76±0.02

3±0.1

0.68

1.79±0.02

0.73±0.01

0.74

5.55±0.10

0.20±0.01

Table 2

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Semiconductor Nanostructures with Efficient Energy

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