Electrically tunable enhanced photoluminescence of semiconductor

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Electrically tunable enhanced photoluminescence of semiconductor quantum dots on graphene M. Praveena, T Phanindra Sai, Riya Dutta, A. Ghosh, and J. K. Basu ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00266 • Publication Date (Web): 28 Jun 2017 Downloaded from http://pubs.acs.org on June 29, 2017

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Electrically tunable enhanced photoluminescence of semiconductor quantum dots on graphene M. Praveena, T. Phanindra Sai, Riya Dutta, A. Ghosh, and J. K. Basu∗ Department of Physics, Indian Institute of Science, Bangalore, India E-mail: [email protected]

Abstract Despite the many fascinating discoveries of fundamental significance and device applications involving graphene one area which has been lacking is graphene based displays and emissive devices. Since graphene by itself has weak and wavelength indpendent absorption and no emission in the visible such devices must rely on synergistic combination with other highly sensitive optical materials like quantum dots. However, the well known strong non-radiative energy transfer between emitters and quantum dots and graphene makes it impossible to create such devices due to strong emission quenching. Here we report, the first demonstration of enhanced photoluminescence of quantum dots closed in proximity to graphene field effect transistor devices which is electrically and spectrally tunable. The enhanced emission originates from superradiance between closely packed quantum dots placed close to a single layer graphene which overcomes the strong non-radiative quenching observed earlier. Finite difference time domain simulations shed light on the regime in which such effects are likely to dominate. Our work opens up new avenues for research on novel displays, lasers and emissive devices involving graphene-quantum dot hybrids as well to study fundamental aspects of electrically tunable light-matter interactions at the nanoscale. ∗

To whom correspondence should be addressed

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Colloidal Semi-conducting quantum dot(CQD) films or layers are being actively studied as novel materials in various applications ranging from imaging, 1–7 efficient displays 8,9 and photo detectors 3,10 to photo voltaics 11,12 and low threshold lasers 13–15 and QD light emitting diodes (LEDs) 12,16 with very high quantum efficiencies. These materials can exhibit very high QE, displaying large spectral tunability and narrow emission even at room temperature while being extremely cost effective. However, the CQDs are usually extremely difficult to dope and have very low electrical conductivity making them relatively inefficient as opto-electronic materials, by themselves. Graphene, on the other hand, is a unique two dimensional (2D) material, which has been extensively studied in the last decade for its extraordinary optical, electronic and mechanical properties. 19–21 It has been shown to exhibit a very high room temperature electrical mobility and hence has been widely used in field effect transistor (FET) devices. 22,23 However, graphene is optically inactive showing no emission and very weak and wavelength independent broadband absorption, especially in the visible part of the spectrum. 24,25 Nevertheless, graphene based hybrid devices have been shown to act as very efficient photodetectors with one of the highest sensitivity and gain achieved in such devices in combination with CQDs 11,26 or other emitters, as shown by us earlier. 27 Such devices combine, synergistically, the large optical absorption cross-section and broadband spectral tunability of CQDs with the high carrier density and mobility of graphene, opening up a new methodology for electrical tuning of light-matter interactions in such grapheneemitter hybrids. 23,28–30 The reason for such high efficiency and sensitivity is the extremely high non-radiative energy transfer (NRET) rate between CQDs and graphene due to the unique 2D nature, especially the gapless linear electronic dispersion, of graphene, leading to a emitter-graphene separation, 11,26,31 d, dependence of NRET of d−4 . If QDs (or quantum emitters) are placed in proximity to metal nanoparticles NRET exhibits d−6 dependence whereas radiative decay rate exhibits d−3 . 6,17,18 Thus NRET dominates the radiative decay rate at small distances leading to strong quenching of emission close to metal nanoparticles but large enhancements can be obtained quite easily at intermediate distances. Although


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NRET is stronger in metal nanoparticles and radiative enhancement with graphene is not possible control of emitter decay rate in proximity to graphene is more attractive due to absence of strong ohmic dissipation, as in metals, and the possibility of electrical tunability of its optical properties, 19,20 which is not convenient with metal nanoparticles. While graphene-emitter interactions has been effectively and widely utilized in various applications, including photodetection and sensing, displays or light emitting diodes (LEDs) based on such hybrid structures are not efficient due to the strong quenching of the CQD or emitter luminescence in close proximity to graphene. 31 If it is possible to device a method to overcome the strong NRET between emitters and graphene then such hybrid devices could also lead to creation of novel displays, LEDs as well as nanoscale optical communication devices based on graphene which could form the backbone for all-optical computers. One such method would be to mediate superradiance (SR) between the CQDs and create conditions under which this SR rate can dominate over the NRET rate. Photonic SR has been demonstrated in CQDs while plasmon mediated SR in CQDs has been predicted and experimentally demonstrated for the first time by us recently. 32 However, plasmon induced SR is a passive method of tuning emission of CQDs whereas electrical tunability of SR between CQDs would lead to a dynamic emission control which could have enormous implications for various photonic devices. Interestingly, although graphene plasmon mediated SR has been theoretically suggested for graphene in the infra-red (IR) regime recently, 33 no experimental demonstration of this effect exists till date, to the best of our knowledge. In this report, we provide the first experimental report of electrically tunable enhanced photoluminescence (PL) of semi-conductor CQDs placed in close proximity to a graphene field effect transistor (FET) device. We demonstrate that this CQD emission enhancement is spectrally apart from being electrically tunable and can also be controlled by varying the packing density of the CQDs on graphene. The radiative decay rate enhancement varies with gate voltage and emission maxima of the CQDs and shows maximum value of ∼ 3 for CQDs having emission maxima at 635 nm (1.95 eV). Finite difference time domain (FDTD)


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Figure 1: Device configuration and electrically tunable grahene band alignment in presence of CQD: (a) Schematic of the CdSe QDs coated graphene FET device fabricated on a highly doped Si/SiO2 substrate. (b) Energy transfer in CQDs due to gate tuning, after laser illumination. (c) Energy level diagram of the graphene and CQD system. Schematic of the system during (d) hole dominated region, (e) charge neutral region and (f) electron dominated region of graphene. Here, VD are the drain voltage and VG is the applied back gate voltage. based electromagnetic calculations clearly illustrate the conditions for emergence of SR and its dominance of over the well known NRET between CQDs and graphene in a regime of CQD lateral sepration of 5 nm or less and for CQD-graphene separation of 3 nm or less. The remarkable results throws open a new field of research on a new generation of optoelectrically tunable graphene-CQD hybrid displays, LEDs and potential all-optical computer devices with possible implications for understanding of the fundamental nature of electrically tunable light-matter interactions at the nanoscale. 34–43


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Results and discussion The schematic of the device structure fabricated for the study is shown in Figure 1(a) (details of the fabrication process are described in the device fabrication section below). The results presented here are based on CdSe CQDs of variable sizes with tunable emission wavelengths of 540nm (SCdSe), 595nm (MCdSe), and 635nm (LCdSe). The respective CQD compact monolayers were transferred onto the SiO2 back gated graphene FET devices using the well known Langmuir-Blodgett (LB) technique. We have verified that this process does not lead to any disruption of the graphene device. The CdSe CQD core was separated from the graphene layer by an insulating capping layer of ∼ 2 nm which is used to stabilise the CQDs. We have shown earlier that such highly compact and dense layer of CQDs are necessary to achieve plasmon mediated SR. 32 Variation of gate voltage, VG , leads to electrostatic doping of graphene and results in control over the Fermi energy, EF , of graphene. This is known to lead to changes in the NRET of the CQDs to graphene which has been shown earlier to enable control over emission of CQDs 19,21,33,44 and illustrated in Fig.1(c-f). However, in all cases reported so far CQDs are always quenched in presence of graphene with VG only controlling the extent of quenching. Further, the NRET, and hence emission quenching, is likely to be more for the LCdSe CQDs as compared to the SCdSe case as per the schematics in Fig. 1(c-f). The resistance vs gate voltage(RVG) characteristics and confocal Raman imaging of the fabricated graphene FET devices were performed in ambient conditions. The electrical and optical characterization were repeated after the graphene FET devices were coated with respective CQDs. Since after coating with the CQD compact monolayer graphene was not visible under the optical microscope, we used Raman imaging to identify regions of the CQD films which were placed directly on top of graphene. Further, we obtained PL spectral maps of the CQDs both on top of graphene as well as those lying on SiO2 outside the gated device to identify the modification of their spectral features due to interaction with graphene. Figure 2(a) shows the optical microscope image of a typical graphene FET device on SiO2 before coating with CQDs with the dotted lines demarcating the graphene flake. The RVG 5 ACS Paragon Plus Environment

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Figure 2: Electrical control of photoluminescence: (a) Optical microscope image of graphene FET device coated with QDs. (b) The RVG curves measured both before and after coating of CQDs on graphene with respect to gate voltage VG . PL image of the sample after coating LCdSe QDs during VG = (c) -20V, (d) +4.5V, (e) +20V and (f) +40V. curves measured both before and after coating of CQDs are shown in Figure 2(b). Hysteresis can be observed in the RVG curves as the measurements were done at room temperature and under ambient conditions. After coating the CQDs the Dirac point shifts to lower gate voltage thereby indicating electron transfer from CQDs to graphene.

Subsequently, gate voltage tunable PL spectral images were collected on these samples and shown in Fig 2 (c)-(f) for a typical LCdSe based FET device. Measurements for the various samples (S/M/LCdSe) were made on separate but identically prepared graphene FET devices and all excitations were performed with 514 nm laser. The outline of the graphene flake is clearly visible in all the images. While Fig 2(c) cearly shows quenched CQD PL on the graphene corresponding to VG of -20V strong PL enhancement seems to be visible for small positive VG . For large positive VG the PL intensity again decreases with a bright CQD region visible just oustide the graphene flake in Fig 2(f). Apart from the overall PL intensity variations at respective VG there seems to be spatial variation of intensity. This


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Figure 3: Electro-optical characterization: PL spectra collected on the graphene FET device coated with (a) LCdSe QD and (b) SCdSe QD monolayer at different gate voltage. (c) Tuning of radiative enhancement, ΓRad , for LCdSe CQDs coated on graphene with respect to gate voltage, VG . Inset shows the same for SCdSe, MCdSe monolayers with identical packing as the LCdSe CQDs and a separate LCdSe CQD monolayer with lower packing density. Here, the PL intensity of CQD is collected at same incident laser power for all measurements. Moreover, the CQD areal density is same on graphene flake as well as on SiO2 of the same substrate.


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Figure 4: FDTD electromagnetic simulations of gate controlled superradiance of CQDs: Schematic of the FDTD model used for simulating interactions between graphene and (a) single and (b) two dipoles. Here, R is distance between two dipoles and Z is separation between a dipole and the graphene surface. (c)Normalized radiative, ΓR , and non-radiative, ΓN R , decay rate of single dipoles (CQDs) on graphene. (d)Co-operative decay rate(Γ12 ) for two dipole case, on graphene, with respect to R. (e)Comparison of decay rates in single and two dipole case with respect to the dipole wavelength at Z = 1.1nm. (f)ΓN R in single dipole case and Γ12 of the graphene system in two dipole case with respect to the chemical potential for different dipole emitter wavelengths.


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is suggestive of heterogeneity of graphene doping along expected lines, 45,46 leading to local fluctuations in the interaction of CQDs with graphene. In fact recent studies 47 have mapped such nanoscale carrier density heterogeneities in graphene with high spatial resolution. To our knowledge, such enhancement of PL intensity of CQDs on graphene compared to that on reference substrate (SiO2 ) has not been observed earlier. To further quantify this remarkable observation we recorded PL spectra from several regions including those on the graphene flake as well as outside. The obtained PL spectra of the LCdSe sample for variable gate voltage, VG , is shown in Figure 3(a). The maximum PL intensity occurs around the Dirac point with the spectral intensity decreasing for increasing positive or negative VG . Clearly, the PL intensity on graphene is significantly higher compared to that on SiO2 for exactly same incident laser power and CQD areal density. For the SCdSe FET devices the PL intensity on graphene is always considerably lower than that on SiO2 irrespective of VG , as can be clearly seen in Fig 3(b), similar to several earlier reports. In order to quantify the PL intensity enhancement or quenching we defined the ratio of the PL intensity, IG , for CQDs on graphene and I0 for CQDs on SiO2 as the radiative enhancement, ΓRad = IG /I0 . This quantity is plotted for all the three CQD based FET devices as a function of VG in Fig 3(c). The large PL enhancement obtained for LCdSe FET devices with a maximum of ∼ 3 is clearly visible along with its gate voltage tunability. The significant PL enhancement observed is smaller but comparable to that observed in our earlier study of SR in similar CQD monolayer films due to plasmon mediated PL enhancement. 32 In that study we also observed that the packing density of the CQDs and the inter-CQD separation influences the observed PL enhancement. If SR is also responsible for the observed PL enhancement then similar effects could also be observed. To check this out we carried measurements on a set of device based on LCdSe CQDs but having a longer capping layer (Octadecanethiol - ODT). Increasing the length of the capping and hence increasing the inter-dot separation leads to significant reduction of PL enhancement; in fact quenching is observed although the gate tunability if retained. The observations indicate possible SR


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effects driving the PL enhancements although it should be noted that graphene plasmons are not expected to be present at the experimental wavelength regime and hence, even if SR exists, its source must lie elsewhere. Further, it is clear that the striking observations of gate tunable PL enhancement of CQDs on graphene in the visible regime cannot be explained by the normal process of NRET that is expected to occur in this regime and has been observed by several others in literature 19,21,33,44,48 and as explained in the schematic in Fig 1. (c-f). Nevertheless, to better understand the observed PL enhancement including its gate voltage and spectral as well as inter-dot separation dependence, we performed FDTD simulations on these systems using a configuration quite similar to that occurring in the experimental FET devices.

In FDTD simulations we, first attempted to model the emission enhancement of dipoles (representing CQDs) placed in close proximity to a graphene layer in terms of the possible radiative rate enhancement of the dipoles due to graphene as compared to their rates in air/vacuum. Figure 4(a) shows the schematic of the geometry used for performing these calculations. Further details are available in SI. 51 As is clear from Fig 4(c) the radiative rate enhancement, ΓR , of the dipole is negligibly small whereas the non-raditive enhancement ΓN R is significantly larger. This is consistent with what has been observed earlier 26,31,33 with CQDs and graphene and is also qualitatively consistent with our observations for the SCdSe and MCdSe based QD-graphene FET hybrid devices. Further, the spectral dependence of ΓN R is consistent with the band digram schematic shown in Fig 1c-f whereby the NRET efficiency, as represented by ΓN R , is expected to become weaker for the SCdSe case (larger band gap and energy mismatch compared to graphene) compared to the LCdSe case (smaller band gap and smaller energy mismatch with graphene energy levels). How can one then explain the observations for the LCdSe CQD based hybrid devices? As alluded to earlier, graphene plasmon mediated SR of quantum emitters has been theoretically predicted to occur in the far IR wavelength regime, 33 although this has not been observed in experiments


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so far. Plasmon mediated SR can lead to strong PL enhancements of compact CQD layers as has been shown by us, experimentally, for the first time recently. 32 Is it possible that the same effect is also responsible for the observed PL enhancement of CQD films on graphene FET devices, as was alluded to earlier despite the fact that graphene plasmons are not expected tobe present for our device geometry and for the wavelength regime used in our experiments? To explore this, we performed further FDTD calculations whereby we studied the decay rate due to coupling of two dipoles, Γ12 , placed near and on the same side of graphene for various distances of their lateral separation, R and vertical separation, z and as explained in Fig 4(b). The decay rate Γ12 is the effect of a dipole Pi placed at ri due to the presence of a dipole Pi placed at rj which is mediated by graphene and is defined in terms of the non-local Green0 s function 33 as,

Γij =

2Ko2 |P~ | ˆ ri , r~j , ω)]~up }. {~up Im[G(~ h ¯ 0

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We have calculated the normalised coupled decay rate Γ12 as the ratio of the co-operative decay rate of two emitters interacting through graphene(Γ12−W G ) to the decay rate of two coupled emitters in absence of graphene (Γ12−W OG ). This rate, is one parameter which is usually used to quantify existence of correlations between two dipoles are separated by some distance R and are otherwise not connected. Hence this parameter would be useful to quantify the extent of SR between two dipole emitters (or CQDs in our experiments) if it indeed exists. Calculations based on other related parameters have been shown in SI (Fig S7). 51 The results of the calculations of Γ12 are quite striking. Figure 4(d) shows the variation of the normalized rate, Γ12 , as a function of R for various emission wavelengths of the dipoles. Strong enhancements of this rate is visible for all wavelengths at R values of 4 nm or less. It is particularly interesting to note that the rate enhancement increases with increasing emission wavelength, for a fixed value of R, and is maximum for wavelengths corresponding to our LCdSe CQD based devices. Expectedly, the


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rate decreases with increasing z. Thus the simulation results clearly identifies the regime where SR between dipoles mediated by graphene can dominate. In our experiments, typical values of R is ∼ 2 nm and z is ∼ 1 nm. These values are therefore perfectly suitable for enabling SR between CQDs on graphene. If SR exists for all the emission wavelengths why do we not observe PL enhancement for the SCdSe and MCdSe cases? To understand this behaviour we now compare the wavelength dependence of both Γ12 and ΓN R . Figure 4(e) shows this comparison for a fixed value of R = 2.2 nm and z = 1.1 nm. Γ12 dominates over ΓN R above 600 nm for the given set of parameters R and z. In fact the simulations predict and even stronger PL enhancement effects at wavelengths of 700 nm. Nevertheless, the simulations explains the mechanism of graphene mediated SR of QDs (Fig 1b) leading to the enhancement/quenching observed with our devices. Increasing R, beyond ∼ 4 nm leads to drastic reduction of Γ12 while decreasing it below this value leads to dominance of Γ12 for a much larger wavelength range. The drastic reduction in experimentally observed PL enhancement for the LCdSe FET devices made with ODT capped CQDs can be explained on the basis of these calculations, especially, those related to reduction in Γ12 for larger values of R. Clearly, the SR becomes stronger in the near field of graphene (see Fig S7 in SI). 51 These results goes beyond our experimental observations and suggests that SR of CQDs on graphene FETs can be quite broadband if one is able to place multiple emitters at very close lateral separations placed in the near field regime of graphene. The emitters seem to stay coupled only in the near field of graphene and at inter-emitter separations of ∼ 5 nm. Since the coupling between emitters in NOT driven by graphene plasmons it is not long ranged as observed in similar studies of plasmon mediated SR in QDs. However, our strategy of creating a close packed CQD solid monolayer in the near field of graphene allows us to overcome the strong NRET of CQDs to graphene by bringing into play the SR effect. If indeed the entire range of predictions from our FDTD simulations, some of which extends into parameter space beyond that achieved in our experiments, is verified in experiments then it opens up enormous opportunities for graphene based opto-electronic


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devices, including the hitherto unobserved high efficiency LED devices with electrically and spectrally tunable properties. Finally, how does one rationalize the VG dependence of the PL intensity variation that we observe in experiments? For this purpose, we have calculated the SR rate for fixed wavelengths and parameters, R and z, as a function of chemical potential, µ, as shown in Fig 4(f). Both ΓN R and Γ12 remains largely independent of µ till µ = Em /2, where Em is the energy corresponding to the emission wavelength of the dipole emitter. For the LCdSe case the drop in Γ12 seems sharper than ΓN R indicating he possibility to tune this system from the enhancement regime to quenching regime by varying chemical potential. While LEDs based on graphene derivations have been demonstrated recently, 49 the external quantum efficiencies are quite low. In principle, our demonstration of enhanced QDPL efficiency by 300% in combination with graphene paves the way for gate-tunable QDLEDs with much improved efficiencies than what is currently available. 8,50 In conclusion, we report the first demonstration of PL enhancement of semi-conducting QDs placed in close proximity to graphene which so far has been widely used as a universal PL quencher of QDs. The PL enhancement is engineered through a unique electrically tunable, wavelength dependent super-radiance between QDs which acts preferentially in the near field of graphene. Our results open up a completely new field of photonic research based on QD-graphene like 2D hybrid devices with potential applications as LEDs and displays, on-chip optical communications and related applications.

Acknowledgement We acknowledge the Department of Science and Technology (Nanomission), India for the financial support and the Advanced Facility for Microscopy and Microanalysis, Indian Institute of Science, Bangalore for the access to TEM and TRF measurements. M.P acknowledges UGC, India for the financial support and RD acknowledges DST for financial support.


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Supporting Information Available Experimental procedures and characterization data for the FET device including details of the electromagnetic simulations are provided in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Graphical TOC Entry Z – separation of graphene and CdSe QD

Laser


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Electrically tunable enhanced photoluminescence of semiconductor

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