Epitaxial Nanoflag Photonics: Semiconductor Nanoemitters Grown

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Epitaxial Nanoflag Photonics: Semiconductor Nanoemitters Grown with Their Nanoantennas Ofir Sorias, Alexander Kelrich, Ran Gladstone, Dan Ritter, and Meir Orenstein Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02283 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on September 1, 2017

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Epitaxial

Nanoflag

Photonics:

Semiconductor

Nanoemitters Grown with Their Nanoantennas Ofir Sorias*, Alexander Kelrich, Ran Gladstone, Dan Ritter, and Meir Orenstein* Electrical Engineering Faculty, Technion - Israel Institute of Technology, Haifa 32000, Israel

ABSTRACT: Semiconductor nano-structures are desirable for electronics, photonics, quantum circuitry and energy conversion applications as well as for fundamental science. In photonics, optical nanoantennas mediate the large size difference between photons and semiconductor nanoemitters or detectors and hence are instrumental for exhibiting high efficiency. In this work we present epitaxially grown InP nanoflags as optically active nano-structures encapsulating the desired characteristics of a photonic emitter and an efficient epitaxial nanoantenna. We experimentally characterize the polarized and directional emission of the nanoflag-antenna, and show the control of these properties by means of structure, dimensions and constituents. We analyze field enhancement and light extraction by the semiconductor nanoflag antenna, which yield comparable values to enhancement factors of metallic plasmonic antennas. We incorporated quantum emitters within the nanoflag structure and characterized their emission properties. Merging of active nano-emitters with nanoantennas at a single growth process enables a new class of devices to be used in nanophotonics applications.

KEYWORDS: nanoantenna, polarization, directionality, dielectric antenna, active antenna.

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TOC One of the important building blocks of nanophotonics are semiconductor nanostructures, employed as light sources or detectors. Low dimensional epitaxially grown nanostructures include nanowires1–3, quantum-dots4, nanomembranes5 and some more exotic nano-trees6,7, nanosheets8,9, nanowings10 and nanosails11. The optical properties of these structures are of great interest and were used in applications such as single photon sources12–14, light emitting diodes15– 18

, nanolasers19–21, detectors22–25, solar cells26, optical communications27, nonlinear effects28 and

more29,30. Another important building block is the optical nanoantenna31. Plasmonic nanoantennas are commonly used in nanophotonics to control the emission pattern32,33 and to enhance lightmatter interactions for light emission34–38, detection39–41, photovoltaics42,43 and more44–46, by changing the local density of photonic states (LDOS)47–50. The metallic antennas are fabricated in the vicinity of the active material, and mediate the electromagnetic energy transfer from the near to far field and vice versa. Typically, fabrication and alignment processes are demanding, the required proximity of the metal-semiconductor interface to the active layers has substantial adverse effects on the device and the metal exhibits optical loss.


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Here we propose and realize the epitaxial growth of both emitter/absorber and the antenna structures with control over their optical properties. We utilize the capability of growing InP nanoflag structures that we recently introduced51 to accomplish this task, obtaining an efficient active optical emitter-antenna epi-structure. We show that the epitaxially grown InP nanoflag-antenna can function as a nanoemitter while mitigating the disadvantages of plasmonic antennas. We start by addressing the nanoflag structure, followed by detailed description of its radiation pattern measurements, mainly the polarization and angular degrees of freedom of the emission. We compare our results to finite difference time domain (FDTD) calculations to support our conclusions. The significant local field enhancement, required for increasing the LDOS, is verified as well. Finally, we realize the incorporation of a quantum emitter into the nanoflag structure and characterize its optical properties. Nanoflag Fabrication. Selective-area vapor-liquid-solid technique52 was employed to grow Wurtzite (WZ) nanowires (NWs) on (111)B InP substrate. Nano gold catalysts were precisely positioned into openings in SiNx mask layer followed by NW growth. Then, by variation of temperature and precursor supply, the catalyst volume was increased on expense of reducing the NW pedestal area and subsequently the catalyst was displaced from the NW top onto one of the six equivalent NW side facets, and migrated along one of the facets until it was pinned by an intentional crystal defect (Figure 1a). On the final stage, a side WZ InP triangular membrane was grown resulting in an array of nanoflags as depicted in Figure 1b. Flag location along the NW is controlled by intentionally introducing a short polytypic segment while growing the initial NW. "Top", “middle” and "bottom" positioned flags are shown in Figures 1c-e. The growth details are described in Ref. 51.


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Figure 1. (a) SEM image showing the migrated catalyst on the NW sidewall. (b) SEM image showing a nanoflag array. (c-e) SEM images of a "top", "middle", and "bottom" flags. All figures were taken at 30˚ angle. The scale bar is 200 nm unless indicated otherwise. Typical dimensions: NW diameter and flag width are 30-60 nm, flag height 14 um, and flag length 0.5-2 um. Triangle head angle is 50.5 degrees.

Spectrum and Polarization. It is well known that III-V semiconductor nanowires and related nanostructures exhibit polarized photoluminescence (PL) depending on their geometry and crystal structure53–55. Here we show that the nanoflag emission is highly polarized, and discuss its features. Polarization resolved PL measurements at room temperature (RT) on individual nanoflags were performed with a micro-photoluminescence setup (see Supporting Figure S1 for details). A linearly polarized 532 nm green laser was focused to ~0.8 µm diameter spot, exciting a single nanoflag (within a 10 µm pitch array of nanoflags). The nanoflag array is depicted in Figure 2a as it was captured by a camera, and the photoluminescence intensity distribution is displayed in the inset. Figure 2b shows a representative polarization resolved PL intensity spectrum with a distinct peak at ~865 nm corresponding to InP WZ structure of the nanoflag56 and a smaller, polarization insensitive, peak at ~925 nm corresponding to the InP ZB structure of the substrate. The nanoflag peak emission is strongly polarized, and the polarization dependencies are shown in Figure 2c for 3 differently oriented nanoflags. The primary polarization angle was consistently found to be parallel to the nanoflags elongation axis as shown in the insets, and due to the hexagonal symmetry of the flag formation, it is oriented on a 60˚


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angular

basis.

The

degree

of

linear

polarization

(DOP),

given

by

DOP = ( Imax − I min ) ( I max + Imin ) , where Imax and Imin stand for the PL intensity polarized parallel and perpendicular to the nanoflag elongation axis respectively, was found to be between 70% and 95%, as indicated in the insets. The polarization axis and DOP values are determined by competing contributions of the optical transition selection rules in the crystal structure and the dielectric mismatch between the nanoflag and surrounding air as in the case for nanowires57,58. Since our measurements were done at RT, the quantum confinement and selection rules have little contribution if any, and the structural properties dictate a parallel polarized emission as it was shown for WZ CdSe NWs59. The DOP values are also influenced by contribution from the original NW (the flag pole) that emits upwards unpolarized light. When measuring the whole nanoflag emission, we also excite and measure some of the original NW light60, which reduces the DOP. Utilizing the high NA of the collecting lens we focused the exciting laser on different positions along the nanoflag, and found not only that the DOP is lower as we get further away from the flag-tip towards the NW, but that it is possible to measure predominant perpendicular polarization, when exciting selectively the NW section bellow the nanoflag triangle, as further explained in the Supporting Section S-B.

Figure 2: (a) Nanoflag array as recorded with camera under white light illumination. The inset shows a single excited nanoflag emitting light. (b) Polarization resolved PL measurement on a single nanoflag. (c) Polar plots of the polarization of the emission for 3 different nanoflags, with DOP values according to cos² fits. The insets show the polarization axis is aligned with flag orientation. (d) Nanoflag PL intensity at 865nm as a function of both exciting laser polarization and emission polarization.


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The polarization dependency of the light-matter interactions in the nanoflag antenna is also evident from the excitation-emission measurements. Figure 2d shows the PL intensity as a function of both exciting laser polarization and emission polarization, advocating the polarized interactions for which this antenna reacts to. For efficient excitation (absorption) of the nanoflag, and effective activation of the antenna, the laser should be polarized parallel to the flag elongation axis. Directivity. When designing an efficient nano-sized antenna, coupled to an emitter, the directivity of the emitted light plays a key role in the characteristics and performance of the device33,61,62. In our case, the emitter within the nanoflag is located above the substrate, resulting in high extraction efficiency, and the directivity aligns the emitted power to a specific optical channel as further explained in the Supporting Section S-D. Here we show the intricate directivity picture of individual nanoflag, analyze it, explain its features, and discuss the control parameters. We show that it is possible to achieve high directionality and we present measurements and simulation results to support our explanations. Angular emission measurements were performed by back focal plane imaging to obtain the emission pattern from a single emitter-antenna. To measure the emission k-space distribution we have added to the micro-photoluminescence setup a Fourier transform lens as further explained in the Supporting Section S-C. Figure 3 shows the measured directivity of 4 differently oriented nanoflags (c-f) and respective FDTD simulation (a). In this case, all 4 flags are located at the top of their NW poles, as shown in Figure 3(b), together with the illustration of the emitted beam wave-vector and angles definition. Two main features are dominant – interference pattern with ‘the image source’ within the substrate (substrate reflection63), and tendency towards the direction of the flag elongation axis due to the triangular shape assisted by reflection from the


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original NW (flag-pole). These features are determined and controlled by the geometrical parameters of the nanoflag antenna, and particularly by the nanoflag height, its position along the NW and the orientation (see Supporting Section S-D for more information). In Figure 3 (g-j) we show directivity measurements of flags positioned at the top of the NW pole for different NW heights. The interference pattern and the number of fringes are determined by the nanoflag height as explained in Supporting Section S-D. FDTD calculation64 results are in accordance with our measurements as shown in Figure S4. Analytical calculations of a simple dipole emitter above a surface can explain the ring-shaped interference pattern, and according to the number and location of the fringes we can extract the nanoflag height as shown in Figure S5.

Figure 3: Directivity images of nanoflags antenna emission. (a) FDTD simulation of the far field emitted from a "top" flag with elongation axis pointing at phi=270˚. The dashed white line indicates the theta and phi angles of the emission. (b) Theta, phi and height definition with wave-vector of an emitted beam from a top nanoflag. (c-f) Directivity measurement of top flags with 2.4-2.8 um height, pointing at different directions. (g-j) Directivity measurements of top flags with different heights. (k) Radiation pattern and its contour as resulted from a simulation of a "middle" flag. (l) Directivity measurement of a middle flag with height≈1um, showing a highly directional emission.

While figure 3 (a-j) refer to "top" flags, "middle" flags positioned in the middle of the flag-pole exhibit reduced number of fringes due to the lower height, possibly, even a single ring. Moreover, in this case the section of the flag-pole positioned above the flag-triangle also reflects the emission, hence the directivity is even more pronounced as shown in Figure 3 (k,l). This pattern resembles the highly directional emission pattern of Yagi-Uda antenna65,66. Figure 3(k)


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depicts the calculated radiation pattern and its contour for such a middle flag as obtained by FDTD simulation relative to its SEM photo. Figure 3(l) shows the directivity measurement of this nanoflag exhibiting highly directive emission, essential for an efficient extraction and transmission of light. We simulated many additional devices, as reported in the Supporting Information. We found that flags with smaller triangle have less azimuthal directionality, however, the flag-pole becomes relatively more influential, resulting in inclination angle confined to a small region in space, as shown in Supporting Information Figure S6. Since the nanoflag geometry is inherently different from a NW, and since the dimensions of the original NW are small, the coupling to NW modes67 is insignificant, and the emission pattern varies fundamentally from that measured for NWs without flags68,69. Moreover, the nanoflag geometry breaks the symmetry found in NWs emission and allows for a more complex radiation pattern. By controlling the geometry of the nanoflag antennas during the growth process, we were able to design its emission pattern, and thus achieve a truly efficient and effective optically active nanoantenna. Highly related to the directivity is the extraction (collection) efficiency to a specific direction or channel or a specific numerical aperture – which is a very important merit for any light source and crucial for single photon sources. We calculated the extraction efficiency of the light into air within a numerical aperture of 0.9, in accordance with our measurements. The extraction efficiency is 50 times higher compared to the same emitters inside a flat InP substrate at close proximity to the surface, and even 2.5 times higher compared to same dipole arrangement in free space above the substrate at the same position as the flag is. This extraction


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efficiency is calculated on top of any emission enhancement effects, and is further explained on Supporting Section S-D. Emission/Field Enhancement. In addition to the unique emission pattern, the emission rate70,71 is expected to be enhanced due to the elevated LDOS of the nanoflag structure – in a similar way to nanoplasmonic antennas50,72. The emission rate enhancement is usually quantified by the Purcell factor73 which is proportional to the quality (Q) factor over the normalized mode volume. For a single nanoflag, our calculation results with Q factor in the range of 15-40, depending on the specific geometry. The Q factor is extracted by the decay time of the energy stored in the nanoflag 'cavity' following an internal dipole emission. The mode volume, which is calculated by the simulated energy distribution according to V = ∫ ε E 2 dV max ( ε E 2 ) , is roughly λ/2 within the flag plane and λ/5 in the dimension confined to the flag width. The resulting Purcell factor, for flags with the above-mentioned typical dimensions, is of the order of ~100, which is similar to values obtained in practice with plasmonic nanoantennas. Smaller flags can support smaller mode volume thus higher Purcell factor. FDTD calculations were performed64 to evaluate the merit of the nanoflag as an antenna by studying its local field intensity enhancement under vertical plane wave illumination. Although not directly analogues to the emission enhancement discussed above, the field enhancement shed some light on the various mechanisms related to emission and absorption in such a dielectric antenna, which may differ from those of a regular plasmonic antenna. The study in this section is focused on the local field enhancement and its resonance, which we define according to the maximum enhancement in the vicinity of the nanoflag tip, just outside the tip, in a similar way to plasmonic nanoantennas. The major field component at this point is normal to the tip, resulting in field intensity that is about ε2~12 times larger than the field


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intensity inside the nanoflag material, which, unlike plasmonic nanoantennas, is the active emitting/absorbing material. The field enhancement and resonant wavelength are greatly impacted by the flag length as detailed in the Supporting Section S-E. It is also strongly affected by the flag height, and in particularly if its position coincides with a constructive interference antinode between the impinging field and its reflection from the substrate. Additional mechanism applicable to arrays of nanoflags is related to the array periodicity. We present three cases that demonstrate and explain these variables. Flags with a triangle length exceeding several hundreds of nanometers, scatter and reflect the light substantially, thus eliminating the effect of interference with the substrate reflection as further described in Supporting Section S-E.

This results in maximum local intensity

enhancement factor, of few tens as shown on Figure S9. Figure 4(a) shows the electric field intensity enhancement profile for this case (nanoflag parameters in the figure caption) resulting in maximum field intensity enhancement of 57 at a wavelength of 1350 nm. Smaller flags scatter less, allowing better utilization of the impinging field and their resonant wavelength is blue shifted. For such a case (details in the figure caption), we got enhancement of over 100 at wavelength of 785 nm as shown on Figure 4(b). As clearly evident, the reflection from the substrate interferes with the incident wave and contributes to the strong local field enhancement at the nanoflag tip. To achieve enhancement factors approaching 1000 we use an array of nanoflags, which, under proper design couple the impinging wave into the nanoflags array, increasing their interaction and achieving higher field enhancement. Figure 4(c) shows a maximum intensity enhancement of 750 at wavelength of 940 nm for a 300 nm long flag at the middle of a 4 um NW arranged in a 0.9 um pitch array.


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Figure 4: Electric field intensity enhancement profile along the nanoflag-antennas cross-section. Incident field is polarized along the nanoflag elongation axis. Nanoflags diameter is 60nm and the other dimensions are: (a) 2um high NW and 0.8um long top flag arranged in a 1um pitch array. Wavelength is 1350nm. (b) 0.2um long flag positioned at the lower part of a 3um high NW. Wavelength is 785nm. (c) 4um high NW and 0.3um long middle flag arranged in a 0.9um pitch array. Wavelength is 940nm.

The large field enhancement for this active dielectric74,75 nanoantenna is comparable to practical values obtained at the vertex of plasmonic nanoantennas40,76. Moreover, the resonant wavelength can be easily designed and controlled during the growth process by varying the flag length, height and array periodicity. By controlling the resonant wavelength, we can influence desirable light matter interaction such as absorption of a pump laser with energy above the banggap, or emission at wavelength appropriate for the InP band-edge, or even enhance the performances of a quantum emitter with lower energy gap incorporated inside the flag. In the latter cases, the nanoflag-antenna performance is upgraded by the negligible material absorption. Quantum emitter incorporation. While the emission so-far was emanated from the "bulk" of the nanoflag, here we embed a lower band-gap material, InAsP, into the InP nanoflag structure to serve as a quantum emitter. The integration of lower bang-gap material into a higher bang gap material forms a heterostructure and increases the efficiency of devices. Such heterostructure was demonstrated, inter alia, in nanowires77, where it may serve as an efficient quantum source78. The InAsP V-shape segment was grown by switching from phosphine to


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arsine precursor flux for 5 seconds during flag growth, in a similar way to the growth of quantum dots in NWs79, and is further described in Supporting Section S-F. The segment structure is shown in Figure S10. The room temperature PL spectrum of the nanoflags with and without the integrated InAsP section is presented in Figure 5(a). In the spectra of the pure InP nanoflags we identify the three main lines at ~850, 870 and 900 nm80, and another peak at ~920-925 nm originating from the ZB InP substrate. The InAsP integrated nanoflag exhibits another prominent peak at ~1000 nm. This strong emission at RT demonstrates the high efficiency and advantage of incorporation of a quantum emitter into the nanoflag in a heterostructure configuration. To examine the properties of the InAsP emission compared to the InP emission from the same nanoflag, we performed polarization measurements of the relevant wavelengths as noted with colored dashed vertical lines in Figure 5(a). Figure 5(b) shows the normalized PL intensity as a function of the polarization angle, for the relevant wavelengths. It is clear that the InAsP emission at 1000 nm has a higher degree of polarization (DOP is 0.78), while the 870 nm and 900 nm has almost the same DOP (0.45, 0.55 respectively), and the substrate emission has the lowest DOP (0.35) which is not zeroed due to the overlap with the broad 900 nm peak of the flag. As expected, the emission originated only from the triangular element of the nanoflag, namely the InAsP emission, has the highest DOP and hence is preferred as the actual active material of the nanoflag antenna for polarization applications.


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Figure 5: (a) PL of a typical InP nanoflag in black doted and dashed, and of InAsP integrated nanoflag in blue. Vertical colored dashed lined represent the relevant wavelength for further investigation. (b) Normalized PL intensity as function of the polarization angle, showing the InAsP emission has a higher DOP. (c) Lifetime measurements showing the InAsP heterostructure longer lifetime.

To further explore the emission properties of this structure we also performed lifetime measurements at RT as shown on Figure 5(c). Using reconvolution methods, described in the Supporting Section S-G, we calculated the lifetime of the nanoflag emission at 870 nm and 900 nm to be ~50 ps. This short lifetime is reasonable considering both the nanoflag surface to volume ratio and the fact that the measurements were done at RT. The lifetime of the substrate emission at 920 nm is ~4 ns, as in the case of bare substrate. The emission from 920 nm and 900 nm are partially overlapping due to their broadened peaks. This causes a lifetime shape of a multi-exponential decay with two decay constants, one of the 920 nm emission and the other of the 900 nm emission, as can be seen in Figure 5(c) and in the Supporting Section S-G. The InAsP heterostructure has a more complex lifetime, and a three exponential decay was found to fit best, with constants of ~50 ps, ~900 ps and ~3.5 ns. The origin of this behavior is a topic for future research, however it is evident that the main InAsP heterostructure lifetime is longer, implying good carrier confinement which makes it more suitable for coherent emission applications or efficient quantum light source. In conclusion, we have presented the nanoflag structure as an epitaxialy grown optically active antenna to be used as an efficient source and detector. We have shown its emission properties which include high DOP, and highly directional radiation pattern that was thoroughly


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explained. Large field enhancement at the flag tip was calculated indicating its potential as efficient optical antenna. The incorporation of lower band-gap material into the InP nanoflag was demonstrated and its emission properties suggest it is highly efficient and advantageous. Altogether, the nanoflag-antenna is a truly efficient active nanoantenna, emitting at the visibleNIR range, opening the way for a new and improved class of sources and detectors with exceptional properties according to specified design.

ASSOCIATED CONTENT Supporting Information. Additional material regarding the following topics is available free of charge via the Internet at http://pubs.acs.org. Polarization resolved micro-PL setup for optical characterization, polarization resolved micro-PL originating from different locations of the nanoflag, back focal plane imaging setup, directivity and interference pattern, local field enhancement simulations, InAsP quantum-wire growth inside InP nanoflag, and lifetime input response function and reconvolution. AUTHOR INFORMATION Corresponding Authors *Email: [email protected] , [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge the partial funding by the Israeli I-CORE program (Circles of Light). This research was supported by Russell Berrie Nanotechnology Institute (RBNI). The fabrication was performed at the Micro-Nano Fabrication Unit (MNFU), Technion. We thank Shimon Cohen for the crystal growth system operation. We thank Dror Miron and Tzoof Hemed for their support on the directivity measurements.

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van Weert, M. H. M.; Akopian, N.; Perinetti, U.; van Kouwen, M. P.; Algra, R. E.; Verheijen, M. A.; Bakkers, E. P. A. M.; Kouwenhoven, L. P.; Zwiller, V. Nano Lett. 2009, 9 (5), 1989–1993. Dalacu, D.; Mnaymneh, K.; Lapointe, J.; Wu, X.; Poole, P. J.; Bulgarini, G.; Zwiller, V.; Reimer, M. E. Nano Lett. 2012, 12 (11), 5919–5923. Gadret, E. G.; Dias, G. O.; Dacal, L. C. O.; De Lima, M. M.; Ruffo, C. V. R. S.; Iikawa, F.; Brasil, M. J. S. P.; Chiaramonte, T.; Cotta, M. A.; Tizei, L. H. G.; Ugarte, D.; Cantarero, A. Phys. Rev. B - Condens. Matter Mater. Phys. 2010, 82 (12), 125327, 1–5.


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TOC 177x127mm (300 x 300 DPI)


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Figure 1. (a) SEM image showing the migrated catalyst on the NW sidewall. (b) SEM image showing a nanoflag array. (c-e) SEM images of a "top", "middle", and "bottom" flags. All figures were taken at 30˚ angle. The scale bar is 200 nm unless indicated otherwise. Typical dimensions: NW diameter and flag width are 30-60 nm, flag height 1-4 um, and flag length 0.5-2 um. Triangle head angle is 50.5 degrees. 150x43mm (300 x 300 DPI)


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Figure 2: (a) Nanoflag array as recorded with camera and white light illumination. The inset shows a single nanoflag emitting light. (b) Polarization resolved PL measurement on a single nanoflag. (c) Polar plots of the polarization dependency for 3 different nanoflags emission, with DOP values according to cos² fits. The insets show the polarization axis is aligned with flag orientation. (d) Nanoflag PL intensity at 865nm as a function of both exciting laser polarization and emission polarization. 201x43mm (300 x 300 DPI)


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Figure 3: Directivity images of nanoflags antenna emission. (a) FDTD simulation of the far field emitted from a "top" flag with elongation axis pointing at phi=270˚. The dashed white line indicates the theta and phi angles of the emission. (b) Theta, phi and height definition with wave-vector of an emitted beam from a top nanoflag. (c-f) Directivity measurement of top flags with 2.4-2.8 um height, pointing at different directions. (g-j) Directivity measurements of top flags with different heights. (k) Radiation pattern and its contour as resulted from a simulation of a "middle" flag. (l) Directivity measurement of a middle flag with height≈1um, showing a highly directional emission. 446x146mm (150 x 150 DPI)


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Figure 4: Electric field intensity enhancement profile along the nanoflag-antennas cross-section. Incident field is polarized along the nanoflag elongation axis. Nanoflags diameter is 60nm and the other dimensions are: (a) 2um high NW and 0.8um long top flag arranged in a 1um pitch array. Wavelength is 1350nm. (b) 0.2um long flag positioned at the lower part of a 3um high NW. Wavelength is 785nm. (c) 4um high NW and 0.3um long middle flag arranged in a 0.9um pitch array. Wavelength is 940nm. 94x80mm (300 x 300 DPI)


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Figure 5: (a) PL of a typical InP nanoflag in black doted and dashed, and of InAsP integrated nanoflag in blue. Vertical colored dashed lined represent the relevant wavelength for further investigation. (b) Normalized PL intensity as function of the polarization angle, showing the InAsP emission has a higher DOP. (c) Lifetime measurements showing the InAsP heterostructure longer lifetime. 190x49mm (300 x 300 DPI)


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Epitaxial Nanoflag Photonics: Semiconductor Nanoemitters Grown

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