Biologically inspired band-edge laser action from semiconductor with dipole-forbidden band-gap transition

A new approach is proposed to light up band-edge stimulated emission arising from a semiconductor with dipole-forbidden band-gap transition. To illustrate our working principle, here we demonstrate the feasibility on the composite of SnO2 nanowires (NWs) and chicken albumen. SnO2 NWs, which merely emit visible defect emission, are observed to generate a strong ultraviolet fluorescence centered at 387 nm assisted by chicken albumen at room temperature. In addition, a stunning laser action is further discovered in the albumen/SnO2 NWs composite system. The underlying mechanism is interpreted in terms of the fluorescence resonance energy transfer (FRET) from the chicken albumen protein to SnO2 NWs. More importantly, the giant oscillator strength of shallow defect states, which is served orders of magnitude larger than that of the free exciton, plays a decisive role. Our approach therefore shows that bio-materials exhibit a great potential in applications for novel light emitters, which may open up a new avenue for the development of bio-inspired optoelectronic devices.

I n recent years, there's a growing interest to develop laser-based photonic technologies, which have made a tremendous impact on modern science and medical applications, such as sensing and diagnosis 1 . Semiconductor nanowires (NWs) possess several unique electronic and optical properties due to the cylindrical geometry and two-dimensional confinement of electrons and holes, which make them particularly attractive as potential building blocks for nanoscale optoelectronic devices, including light emitting diodes and lasers 2 . As an active photonic device, lasers are composed of a pumped gain medium placed within an optical resonator. Based on these two elements, semiconductor nanowires not only serve as a gain medium but a cavity as well for laser action. Simultaneously, the large refractive index of semiconductors and wire-like geometry enable strongly scattering process existed in the NWs system, and the constructive interference effect consequently results in optical coherent feedback for the outcome lasing modes. Room-temperature lasing has been readily achieved in ZnO, GaN, GaAs and CdSe nanowires due to their large exciton binding energy [3][4][5][6][7] . Fruitful derivatives are also promising in wide fields such as biosensors, transistors, gas sensors and solar cells [8][9][10][11] . Among the functional wide-bandgap semiconductors, only limited progress has been made in the field of ultraviolet tin dioxide (SnO 2 ) nanowire lasers. Although SnO 2 has a wide direct bandgap (3.6 eV) and large exciton-binding energy (130 meV), it is so far commonly believed that SnO 2 is not a suitable ultraviolet (UV) luminescent material due to the dipoleforbidden nature of its bandgap quantum states 12 . Generally, only a broad visible oxygen defect photoluminescence (PL) is observed in most of the reports, since the even-parity symmetry of the conduction band and valence band states in SnO 2 prohibits the bandgap radiative transition 12 . Until now, only a little attention has been paid to the research for bringing up the related mechanism of SnO 2 photoemission in ultraviolet range, the underlying origins still remain as an issue of debates 13,14 . More theoretical insights are still in need. However, in order to achieve a better UV performance of SnO 2 NWs and overcome the difficulties for future coming applications, an improvement for the external conversion efficiency is essential and now paramount.
Chicken egg white (albumen), as a kind of ubiquitous nutritious food in our daily life can serve as a new excellent UV-emitting (,340 nm) protein. Compared with traditional laser dyes (e.g. rhodamine 6G (R6G)), the advantages of chicken albumen consist of the biodegradable, bioresorbable and biocompatible characteristics. More attractive, it is cost-effective and environmentally friendly. Very recently, related research has been paid to the field of modern organic optoelectronics, such as light emitters and field effect transistors (FET) 15,16 . As a new generation of efficient emitting biomolecule, chicken albumen is expected to be potential for future green technologies.
In this paper, we demonstrate, for the first time a facile and practical strategy to fabricate a newly designed organic-inorganic hybrids composite based on chicken albumen/SnO 2 NWs. Quite surprisingly, it is observed that the originally undetectable UV fluorescence (387 nm) in pristine SnO 2 NWs can be drastically enhanced in the albumen/SnO 2 NWs composite. More excitingly, via effective optical excitation, the coherent laser action can be further sustained and derived. Our strategy to overcome the UV-light-emitting drawback of SnO 2 NWs is based on the fluorescence resonance energy transfer (FRET). FRET is a physical phenomenon where excitation energy from an excited donor (chicken albumen) is non-radiatively transferred to a proximal ground-state acceptor (SnO 2 NWs), and the energy transfer process in a FRET system requires good overlap between optical transition bands of donor and acceptor as well 17 . As reported, FRET has been shown to be sufficiently applicable to a variety of biological systems 18 . Our proposed unprecedented FRETassisted UV-emitting laser device based on albumen/SnO 2 NWs composite is not only simple but also paves an excellent alternative way of utilizing chicken albumen protein, which may enable to trigger the further development of bio-inspired optoelectronic devices.

Results
Morphology and optical characterization of SnO 2 NWs and chicken albumen. Figure 1(a) shows the top view SEM image of the as-synthesized SnO 2 NWs. It is observed that the nanowires are randomly assembled and closely packed. An inset of Fig. 1(a) reveals a closer view on SnO 2 NWs. The average length is of about 10 mm, while the diameter is ranging between 70 nm and 150 nm. The XRD pattern of the as-prepared SnO 2 NWs is shown in Fig. 1(c). It can be seen that all peaks are perfectly indexed to the tetragonal rutile SnO 2 . As additional evidence, a Raman scattering spectrum is shown in Fig. 1(d), in which the three peaks at 475, 630, 768 cm 21 correspond to the E g , A 1g , and B 2g vibration modes, respectively. All these informations further confirm the existence of the as-grown SnO NWs. For the optical properties, a PL spectrum is first characterized as shown in Fig. 1(b). We can clearly see that only a broad orange emission peak located at 625 nm (2.0 eV) is observed, and no ultraviolet fluorescence can be detected. The detected visible light arising from SnO 2 is generally believed to stem from the deeptrapped state, which is related to the oxygen vacancies (O v ) or tin interstitials (Sn i ) 14 . Figure 2 presents the PL spectra for both of the pristine SnO 2 NWs and chicken albumen. The UV emission arising from albumen is centered at around 340 nm, and the FWHM is much narrower compared with that of the bare SnO 2 NWs. It is found that the optical property for albumen is quite stable even under the UV laser pumping. At the beginning, the albumen was spin-coated on a cleaned glass substrate for the PL measurement. The first sample was then stored and preserved in a Petri dish at room temperature. More than our expectation, the PL can still be detected and shown to be stable even after 6 months. Based on these characteristics, albumen from chicken egg reveals one of its advantages as an excellent UV-emitting biomolecule. The inset of Fig. 2 illustrates the separation of egg white (albumen) and egg yolk. In addition to the fluorescence properties, the transmittance spectra of albumen with different spin-coating speed were also shown in Fig. 3. The film thicknesses are about 800 nm (1000 rpm) and 400 nm (5000 rpm), respectively. Both the albumen samples with different thickness show similar transmission properties which indicate good transparency from ultraviolet to visible range. The inset of Fig. 3 shows the topological AFM of the albumen surface, and the root mean square (RMS) roughness is 0.34 nm.
Photoluminescence and laser action of albumen/SnO 2 NWs composite. Figure 4 shows the PL spectrum of the organicinorganic hybrid structure of albumen/SnO 2 NWs composite. At the beginning, no ultraviolet fluorescence can be ever detected   from the as-synthesized bare SnO 2 NWs. However, to our astonishment, when chicken albumen is spin-coated (5000 rpm) onto the SnO 2 NWs, two UV emission peaks located at the positions of 340 nm and 387 nm emerge. Similar result can both be found in high-density ( Fig. 4(a)) and low-density ( Fig. 4(b)) grown SnO 2 NWs coated by albumen. The 340 nm emission is indexed to the existing albumen covered on SnO 2 NWs, and is basically the same as prior shown in Fig. 2. Compared with the albumen emission, we are more intriguing to characterize the new emerging 387 nm UV fluorescence. Tin dioxide is a direct bandgap semiconductor, however, limited by its dipole-forbidden nature, the bandgap emission with the photon energy of 3.6 eV (,344 nm) is prohibited at room temperature due to the selection rule 14 . In addition to the former described deep-trapped state (,2.0 eV) resulting in the visible light, a shallow-trapped state (3.2 eV) within the bandgap of SnO 2 has also been found 14 . Via the comparison of Figs. 4(a) and 4(b), it is apparent that the photoemission of 387 nm strongly depends on the density of SnO 2 NWs, and decreases as the NWs turn to be sparse. Herein, the observed UV fluorescence centered at 387 nm (3.2 eV) is believed due to the shallow state of SnO 2 NWs as shown in the early reports 4,13,14 . To test the photo-stability of the emission of 387 nm from the albumen/SnO 2 composite, the PL intensity was measured at regular time interval under a persistent laser pumping with fixed energy (left inset of Fig. 4(a)). All the recorded data remained in the same order of magnitude with a considerable stability of more than 600 sec. Therefore, an excellent photo-stability of our sample is further confirmed for practical applications. It is worth noting that the visible defect emission of SnO 2 NWs remains nearly the same before and after albumen coating. Since the visible emission is mainly due to the oxygen vacancies and independent of the albumen coating, the emission spectra are therefore not shown here. The rest right insets of Fig. 4 show the SEM images of dense and sparse SnO 2 NWs without albumen coating. Concerning that the SnO 2 NWs are randomly assembled, the coating of albumen might be partially uneven. However, supported by the transmittance spectra in Fig. 2, the similar fine transparency of albumen both for 1000 rpm and 5000 rpm indicates that the thickness effect on the intensity of the photoemission (387 nm) can be fairly ignored.
In order to interpret the enhancement of 387 nm UV emission, the fluorescence resonance energy transfer (FRET) is believed to be the most possible underlying mechanism. For the occurrence of FRET, donor should be located in the close proximity of acceptor. Meanwhile, a good overlap is required between the optical transition bands of donor and acceptor. In our study, the donor and acceptor are albumen protein and SnO 2 NWs, respectively. According to the previous reports 15,19,20 , the absorption spectra of albumen and SnO 2 exhibit a good overlap ranging from 250 nm to 300 nm. It is well documented that the strong optical absorption in albumen is at around 3.65 eV, while that of SnO 2 occurs at 3.6 eV. Therefore, the newly designed albumen/SnO 2 NWs composite is well feasible for the proposed FRET mechanism, in which the energy can be efficiently transferred from albumen protein to SnO 2 NWs. Figure 5 illustrates the band alignment diagram, which provides a clear physical picture to describe the FRET process between chicken albumen and SnO 2 NWs via two dominant steps. First, in Fig. 5(a), due to the good overlap between the absorption bands of albumen and SnO 2 , the energy absorbed by the albumen protein can be easily transferred to SnO 2 NWs through the resonant FRET effect. Second, the 340 nm UV transition arising from Tryptophan in albumen also shows an excellent overlap with the bandedge transition of 344 nm in SnO 2 NWs thus supports the origin of FRET as well. However, limited by the direct-forbidden nature, the bandgap emission with the photon energy of 3.6 eV is prohibited. Therefore, the excited carriers will transfer to the nearest shallow-trapped state and result in the UV fluorescence with the wavelength of 387 nm (3.2 eV). Based on the above proposed mechanisms, the PL spectra shown in Fig. 4 can now be well interpreted.
Lasing occurs in a cavity by stimulated emission, which provides optical amplification 1 . Herein, the coherent optical transition within the bandedge of gain medium is important. For the occurrence of deep-trapped recombination in SnO 2 NWs, the long decay time makes it not capable to block the fast coherent radiation of bond excitons. On the contrary, the excited carriers are much more efficient to bind and populate into the shallow trapped states, and emit photons with a fast decay rate 14 . Hence, the laser action amplified by coherent stimulated emission can be expected in the albumen/SnO 2 NWs system at the wavelength of 387 nm rather than the visible range. To confirm our expectation, the albumen/SnO 2 NWs composite were optically pumped by the Q-switched Nd: YAG laser (266 nm, 3-5 ns pulse, 10 Hz) for the lasing study. Series of fluorescence spectrum under different excitation energy were performed as shown in Figs. 6(a)-(d). At a low pumping power such as 40 mJ, the PL spectrum is broad and featureless ( Fig. 6(a)). However, when the pumping energy exceeds a specific threshold of 45 mJ (Fig. 7), a sharp lasing peak at around 387 nm with the line width (FWHM) less than 1.3 nm starts to emerge. The rapid increase of fluorescence intensity indicates that the stimulated emission occurs in the hybrid system. As the pumping energy goes higher, the more output lasing modes appear around the wavelength of 387 nm. It is worth noting that the albumen emission located around 340 nm shows a reduction as the lasing peaks and intensity grow. As additional evidence, the reduction further supports the strong energy transfer from albumen protein to SnO 2 NWs via the proposed resonant FRET effect. Discussion For the occurrence of laser action in our study, Fabry-Perot (FP) and Whispering Gallery Mode (WGM) types will not be considered as main mechanisms, since the former requires highly oriented vertical nanowire array providing flat facets in both ends as an optical cavity, while the later needs spherical or hexagonal shape cavity as a confinement of light 21,22 . It is believed that the dominant mechanism can be interpreted by the random lasers (RLs). RL consists of a randomly distributed structures dispersed into an optical gain medium, in which the closed-loop path (optical cavity) is merely provided by multiple scattering process of light 23 . Therefore, the randomly assemble tetragonal SnO 2 NWs are well feasible for the occurrence of RL. By using the information of the wavelength difference (Dl) derived from the two nearest lasing peaks, the scattering mean free path (L) of a light in the composite system can be calculated by L 5 l 2 /2nDl 24 , where l is the resonant wavelength (,387 nm), n is the refractive index (,2), L is the resonant cavity length, and the approximate value is about 23.4 mm. Besides, the varied lasing modes and intensity under different pumping further confirm the RL existence in the albumen/SnO 2 NWs composite, since the random cavity made by closed-loop path changes every moment. It should be noted that RL derived from the albumen/SnO 2 NWs composite contains only few peaks compared with the early reports 4 . This may be explained by the reduction of scattering strength since the refractive index of albumen (n , 1.35) is less than that of the SnO 2 NWs (n , 2.0) 25 , as well as by the narrow spectrum due to the shallow trapped state 14 . The advantage of mode reduction leads to the possibility of mode controlling and towards mode locking for the future RL applications. The emission peak intensity as a function of pumping energy is shown in Fig. 7, from which the threshold (P th ) of about 45 mJ can be derived. The inset of Fig. 7 illustrates the closed-loop path inside nanowires. It is intriguing to further realize why the shallow-trapped  state caused by impurities can produce the strong stimulated emission. In 1962, Rushba et al. first pointed out that the weakly bond exciton of impurities or defects can generate a giant oscillator strength, which is many orders larger than that of the free exciton 26 . The wave function generated from the weakly bond exciton basically involves a whole region around defects, thus leads to the coherent oscillations of electrons 14,26 . On the shallow-trapped state, there are many connected bond excitons 26 , it is therefore believed that the stimulated emission can be well generated. This theoretical work provides an excellent foundation for the understanding of our observed behavior that the energy harvesting from FRET process can be efficiently turned into the UV emission arising from shallow defects in SnO 2 NWs.
In conclusion, we have demonstrated that a giant enhancement of 387 nm UV emission can be easily achieved from SnO 2 NWs by the assistance of UV-emitting (340 nm) chicken albumen protein at room temperature. The underlying mechanism is interpreted by the fluorescence resonance energy transfer (FRET), in which the energy can easily transfer from albumen protein (donor) to SnO 2 NWs (acceptor). Laser actions can be further derived from the albumen/SnO 2 NWs composite as well. Via a careful characterization, the laser action is believed to be the RL type lasing and is generated by the stimulated emission arising from the shallow-trapped state of SnO 2 NWs. The efficient transfer of energy harvesting from FRET process into UV emission is supported by theoretical work of extremely high oscillator strength of shallow defects in SnO 2 semiconductors. It is believed that our work shown here not only enables to open up the possibility of utilizing various biomolecule options for improving wide fields of light emitters, but also pave a new avenue towards future green bio-inspired optoelectronic devices.

Methods
Sample fabrication. The n-type SnO 2 nanowire arrays were grown on silicon (Si) substrates via vapor -liquid-solid (VLS) method. Prior to the growth, the single crystal Si substrates (0.5 cm 3 0.5 cm) were ultrasonically cleaned for 10 min in acetone, ethanol and deionized (DI) water to remove any absorbed contaminant. Next, Au thin film with a thickness of 10 nm was deposited on Si substrates by using a sputtering system (JFC-1600, JEOL). A high purity (99.99%) Sn metallic powder (1.5 g) was placed on a ceramic boat and the Au-coated Si substrate is nearby the powder. The boat was then loaded to the center of a horizontal tube furnace under the Argon gas with a flow rate of 200 sccm (sccm denotes cubic centimeter per minute at STP). After the above steps, the furnace temperature was elevated to 1000uC rapidly at a rate of 100uC min 21 . The sample was kept annealing for 10 min at 1000uC. During the process, the vaporized Sn was reacted with oxygen and resulted in SnO 2 which blown onto the Au layer. As SnO 2 dissolved into Au, the nucleation occurred when the alloy of SnO 2 -Au droplet reached supersaturation. After the furnace cooled down to room temperature, the as-grown white color SnO 2 NWs were obtained on Si substrate. Albumen liquid was obtained from chicken eggs purchased from a widely seen convenience store. After separating the egg yolk, the rest albumen liquid was directly used without any further purification or post baking. To fabricate our albumen-coated SnO 2 NWs composite, the extracted albumen was spin-coated (5000 rpm) onto the as-grown SnO 2 NWs for 60 sec.
Morphology characterization and optical measurements. The morphologies of SnO 2 NWs and surface roughness of albumen were characterized by scanning electron microscope (SEM; JEOL JSM6500) and atomic force microscopy (AFM; Nanosurf Easyscan 2). The existence of SnO 2 NWs were confirmed by the Raman scattering measurement (Jobin Yvon T64000), X-ray diffraction (XRD; Panalytical X'pert PRO), and PL spectra were carried out by the excitation of 266 nm laser. The transmittance spectra of albumen with different spin-coating speed were detected with a spectrophotometer (Jobin-Yvon H10). To study the lasing behavior, the albumen-coated SnO 2 NWs were optically excited by a Q-switched 4 th harmonic Nd: YAG laser (266 nm, 3-5 ns pulse, 10 Hz) and measured with a Jobin Yvon iHR550 imaging spectrometer system. The laser beam was focused to a diameter of about 300 mm. All of the luminescence experiments were performed at room temperature.