Nonequilibrium hot-electron-induced wavelength-tunable incandescent-type light sources Download: 954次
1. INTRODUCTION
Nonradiative decay of surface plasmons generates hot carriers in metal nanostructures that can be injected into neighboring semiconductor micro/nanostructures. Such working principle on the generation of hot carriers has been utilized to construct electronic and optoelectronic devices, such as solid-state lighting and displaying, photodetection devices, and photovoltaic and photocatalytic devices [1
In addition to optical illumination, energized electrons have also been employed to excite metal plasmons, such as high-energy free electrons (
In this work, wavelength-tunable incandescent-type light sources composed of individual Ga-doped ZnO microwire (MW) deposited with Au nanorods were constructed (AuNRs@ZnO:Ga MW). Especially by adjusting the aspect ratio of the Au nanorod, wavelength-tunable near-infrared light-emission can be achieved, with the dominating emission peak ranging from the visible to near-infrared spectral band. To exploit modulation of Au-nanorods on the emission characteristics, it could be found out that Au-nanorod plasmons induced generation and injection of nonequilibrium hot electrons can be responsible for the wavelength-tunable incandescent-type lighting features. Furthermore, to demonstrate the scalability and practicability of the incandescent-type emissions, size-dependent ZnO:Ga MWs were also selected to construct distinctive fluorescent filament emitters by introducing Au nanostructure decoration factitiously. Therefore, by means of the accurate control over the sizes of as-synthesized ZnO:Ga MWs, aspect ratio of Au nanorods, and the deposition of the Au nanorods on the ZnO:Ga MWs, single AuNRs@ZnO:Ga MW can provide a key building block in solid-state lighting and displaying. In particular, it also affords an inspiring platform for fundamental studies and technological applications in the fabrication of multicolor light sources and light-emitting diodes.
2. EXPERIMENTAL SECTION
2.1 A. Synthesis of Individual ZnO:Ga MWs
Individual ZnO:Ga MWs were successfully synthesized via the chemical vapor deposition (CVD) method [37–
2.2 B. Synthesis of Au Nanorods with Controlled Aspect Ratios
Well-defined Au nanobipyramids (NBPs) with controlled sizes were employed to prepare Au nanorods with desired plasmonic properties and heat stability [4143" target="_self" style="display: inline;">–
2.3 C. Device Fabrication
A single ZnO:Ga MW was selected, and then transferred onto the quartz substrates. Two indium (In) particles serving as the electrodes were applied to fix the MWs, leading to the fabrication of light-emitting devices based on the metal–semiconductor–metal (MSM) structure. Specifically, Au nanostructures were deposited on the MWs by spin coating. After annealing for 30 min at 150°C, hybrid architecture composed of a single ZnO:Ga MW prepared with Au nanostructures decoration was fabricated.
2.4 D. Analysis Instruments
The size, surface morphology, and crystal structures of single ZnO:Ga MWs prepared via Au nanostructures were characterized using a scanning electron microscope (SEM). Au nanorods were also characterized using a transmission electron microscope (TEM). To confirm that Ga was incorporated into ZnO MWs, elemental mapping distribution of Zn, Ga, and O species was also characterized using energy-dispersive X-ray spectroscopy (EDX). The current–voltage (
3. RESULTS AND DISCUSSION
As previous literature reported, individual ZnO MWs with controlled Ga incorporation have been utilized to construct wavelength-tuning semiconducting lamp filaments [19,44,45]. Meanwhile, owing to the outstanding optical behaviors, chemical stability, biocompatibility, and high-temperature tolerance, Au nanorods with controlled aspect ratios have been widely applied in the applications of photovoltaic and photocatalytic devices, photodetectors, and so on [7,17,41,42,46]. Herein, a typical semiconducting filament emitter composed of single ZnO:Ga MW prepared with Au nanorods decoration was fabricated, and the experimental fabrication procedure was schematically described in the experimental section. During the fabrication procedure, Au nanorods with an extinction peak centered at 695 nm were selected, as illustrated in Fig.
Fig. 1. EL emission characteristics from single AuNRs@ZnO:Ga MW-based incandescent-type light source (the extinction peak of Au-nanorod, 695 nm). (a) The extinction spectrum of Au nanorods, with corresponding TEM image of the Au nanorods demonstrated in the inset. (b) SEM image of single AuNRs@ZnO:Ga MW. (c) Amplified SEM image of Au nanorods deposited on the MW. (d) characteristics of single ZnO:Ga MW prepared via Au nanorods decoration. (e) EL emission spectra from single ZnO:Ga MW-based fluorescent emitter. (f) EL emission spectra from single AuNRs@ZnO:Ga MW-based incandescent-type light source. (g) Optical microscopic image of the light emitting from electrically biased single ZnO:Ga MW-based fluorescent emitter (scale bar, 200 μm). (h) Optical microscopic image of the light emitting from electrically biased single AuNRs@ZnO:Ga MW-based incandescent-type light source (scale bar, 200 μm).
Especially when the injection current exceeded a certain value, such as 8.5 mA, green lighting can be observed, with the emission regions located at its center. Increasing the injection current can enhance its brightness, as well as expand the lighting regions. To further characterize the emission characteristics, Fig.
Considering the influence of Au nanorods on the modulation of typical near-infrared emissions, optical and electronic properties of single AuNRs@ZnO:Ga MW were measured. First, single ZnO:Ga MW with a quadrilateral cross section was synthesized via one-step carbothermal reduction methods. Figure
Fig. 2. (a) Optical photograph of the synthesized ZnO:Ga MWs. (b) SEM image of single ZnO:Ga MW, with perfect quadrilateral cross section displayed in the inset (scale bar, 12 μm). (c) SEM image of ZnO:Ga MW prepared with Au nanorods decoration (the spin-coating number, ×1). (d) TEM images of the Au nanorods with controlled aspect ratio. (e) The extinction spectra of Au nanorods with controlled aspect ratio. (f) PL emissions from ZnO:Ga MW prepared via Au nanorods decoration, with the controlled aspect ratios. (g) behaviors of single ZnO:Ga MW prepared with Au nanorods decoration (corresponding extinction peak centered at 695 nm), with the spin-coating number ranging from 0 to 6.
Room-temperature PL measurements were carried out on account of single ZnO:Ga MW deposited via Au nanorods decoration. In the case of PL measurement, a single ZnO:Ga MW with the length up to 2 cm was selected, and then divided into four segments. PL emission of single bare ZnO:Ga MW displayed that ultraviolet emission centered around 378 nm dominated the PL spectra, accompanied with weak visible emission, as illustrated in Fig.
To probe into influence of Au nanorods on the electronic transport properties, the deposition of Au nanorods on the MW was also taken into account by adjusting the spin coating. Again, the
While introducing light illumination, the photoconductive behavior from single AuNRs@ZnO:Ga MW was characterized. The relevant schematic diagram is depicted in Fig.
Fig. 3. Photoconductive behavior of single AuNRs@ZnO:Ga MW [Au nanorod in Fig. 2(d) panel II, the extinction peak, 695 nm]. (a) Schematic diagram of hot carrier generation mechanisms in plasmonic Au nanorods, and then injected into ZnO:Ga MW channel under light illumination. (b) The characteristics of single ZnO:Ga MW prepared with Au nanorods decoration under dark, and illumination with the excitation lasing wavelengths at 405 nm, 532 nm, and 685 nm, respectively, with the laser power density denoted as . (c) The characteristics with on/off switching under light illumination, with the lasing wavelengths at 405 nm, 532 nm, and 685 nm, respectively. (d) UV-vis absorption spectra of the as-synthesized ZnO:Ga MWs prepared with and without Au nanorods deposition. (e) The comparison of TRPL decays from single bare ZnO:Ga MW, and Au nanorods decorated ZnO:Ga MW. (f) Diagrammatic drawing of the physical process involving (i) photoexcitation induced electrons and (ii) plasmons induced generation, injection, or tunneling procedure of hot electrons towards the interface between Au-ZnO:Ga under light illumination.
In addition, the time-dependent photocurrent curves of single AuNRs@ZnO:Ga MW (the extinction peak, 695 nm) with on/off switching upon light illumination at zero bias voltage are shown in Fig.
Afterwards, the typical absorption of single ZnO:Ga MW prepared with and without Au nanorods deposition was carried out, as shown in Fig.
It is well-known that the hot carrier generation from metal nanostructures can be divided into two major physical mechanisms: direct photoexcitation and nonradiative plasmonic decay [60]. Direct photon absorption from metallic nanostructures could be utilized to produce a photocurrent; nevertheless, due to the small electron–photon cross section, the hot carrier generation is very inefficient. Additionally, the photoexcited carriers from Au nanorods are primarily dominated by interband transitions. Under lighting illumination, electrons can be excited, and then transited from the d-band with its upper edge
To exploit the generation mechanism of photoinduced carriers from Au nanorods, a diagrammatic drawing is depicted in Fig.
Full-field electromagnetic simulations were also carried out using a commercially available software package, Lumerical FDTD. The metallic particles in this study are assumed to be Au nanorods, and are modeled by using experimental dielectric data [17,62]. During the simulation procedure, the Au nanorod with the length (50 nm) and diameter (20 nm) was adopted, accompanied with the resonant wavelength denoted as
Fig. 4. Electrical field intensity distribution of isolated Au-nanorods, with the electromagnetic wave propagating along (a) the direction of the plane (horizontal), (b) the direction of the plane (vertical), and (c) the direction of the plane (horizontal). In the simulation process, Au-nanorods with the length (50 nm) and diameter (20 nm) were adopted, accompanied with the resonant wavelength denoted as , the refractive index of ZnO:Ga denoted as , and the refractive index of environmental medium air .
As we previously stated above, Au nanorods with the extinction peak centered at 695 nm can be utilized to construct near-infrared incandescent-type light sources from electrically biased single AuNRs@ZnO:Ga MW based incandescent-type lamp filament. Meanwhile, the dominant emission peaks relate to the plasmons of Au nanorods, accompanied with strong electric field localizations towards the deposition areas of Au-nanorods. Additionally, PL emission measurements from single AuNRs@ZnO:Ga MW also demonstrated that the enhancement of the ultraviolet emission can be attributed to cooperative effect of the suppression of the visible emission and Au-nanorod plasmons induced hot electron generation and injection from Au nanorods into the conduction band of ZnO:Ga [53,54,63]. Due to the mismatching between the extinction peaks of Au-nanorods and the negligible PL emission in the near-infrared spectral band of single MW, the electrically driven near-infrared emissions cannot be attributed to Au-nanorod plasmon selectively amplified near-infrared emission from single bare ZnO:Ga MW. To exploit the remarkable modulation of near-infrared emission behavior from an electrically driven single AuNRs@ZnO:Ga MW based filament-type light source, a diagrammatic drawing is depicted in Fig.
Fig. 5. (a) Schematic illustration of the modulation of Au-nanorod plasmons on the incandescent-type lighting features of single ZnO:Ga MW-based fluorescent light source. (b) Normalized intensities of the EL spectrum from single bare ZnO:Ga MW-based fluorescent light source, the EL spectrum from single AuNRs@ZnO:Ga MW-based fluorescent light source, and the extinction spectrum of the deposited Au nanorods. (c) Micrographs of bright visible light emitting from an electrically driven single AuNRs@ZnO:Ga MW-based incandescent-type light source in the dark field and bright field (scale bar, 200 μm). (d) Optical microscopic images of bright visible light emitting from electrically driven single ZnO:Ga MW prepared with partial Au nanorods decoration (scale bar, 300 μm).
Despite the excitation energy sufficiently large above the Fermi level and thus the nonradiative plasmonic decay, with the relaxation time in the order of the nanosecond to millisecond rate, nonequilibrium hot electron distribution can be produced [8,10,16,66]. However, electrically driving the generation of nonthermalized hot carriers is still at the primary stage. That is, to explore the likelihood of electrically driving the generation of nonequilibrium hot electrons from single AuNRs@ZnO:Ga MW, normalized intensities consisting of the extinction spectrum of Au nanorods, EL emission spectrum of the single bare ZnO:Ga MW-based emitter, and EL emission spectrum from single AuNRs@ZnO:Ga MW-based emitter were depicted, as shown in Fig.
To further exhibit the modulation of Au-nanorods on the emission characteristics of a single ZnO:Ga MW-based filament-type emitter, a single ZnO:Ga MW with the length
The incandescent-type lighting working mechanism is depicted in Fig.
Fig. 6. Schematic diagram of the working principle of bright visible light emitting from electrically biased single AuNRs@ZnO:Ga MW-based incandescent-type light source.
To probe into the near-infrared incandescent-type emission features, it can also be found out that the hot carrier transfer rate is being higher than its relaxation rate in the single AuNRs@ZnO:Ga MW, accompanied with the nonequilibrium hot electron in the conduction band with its energy up to plasmon frequency. After relaxation of the nonequilibrium hot electrons, another energy level may be created in the energy-level configuration of ZnO:Ga, resulting in the thermal and nonthermal distributions overlap in population. Therefore, during this relaxation, fractional electrons could be scattered into energy-level configuration of ZnO:Ga (luminescent states), where they can recombine with holes in the form of emitting photons. It is reasonable to deduce that the dominating emission wavelength of the emitted photons is relevant to the energies of the hot electrons, and then redshifts monotonically after the relaxation procedure. The generation and injection of hot electrons with energy up to the plasmon frequency can be utilized to dominate the characteristic of the emitted photons, such as the emission wavelength and spectral linewidth [8,27,30,66]. Despite that, Joule heating effect played a significant role in the incandescent-type emission characteristics, but it cannot be compatible with the thermal radiation mechanism (blackbody radiation theory). By comparing the emission spectra collected from the single AuNRs@ZnO:Ga MW-based incandescent-type light source and the corresponding emission peaks from the theoretical models of blackbody radiation, the maximum temperatures were calculated and can approach 3000 K, which far more greatly exceeded the endurance of single ZnO:Ga MW [68,69]. In addition, for the emission characteristics, such as wavelength-tuning emission from individual ZnO:Ga MW-based light sources heavily depending on the Ga-incorporation concentration, increasing the injection current can bring a little redshift of the dominating emission peaks, while reducing the ambient temperature can bring significant enhancement of the EL emissions and so on, and can also rule out the thermal radiation mechanism. Therefore, there is no direct experimental evidence that the incandescent-type lighting characteristics, such as the emission wavelengths, spectral linewidth, and the incandescent-type lighting output intensity, were dominated by the temperature generated by the Joule heating effect [19,45,69,70].
By introducing Au nanorods with controlled aspect ratio deposition, a near-infrared filament-type light source composed of individual ZnO:Ga MWs was fabricated. Taking single ZnO:Ga MW prepared via Au nanorod [panel I in Fig.
Fig. 7. Wavelength-tunable emissions from single AuNRs@ZnO:Ga MW-based incandescent-type light source: (a) characteristics of single ZnO:Ga MW via Au nanorods decoration (the extinction peak, 605 nm); (b) EL emission from single bare ZnO:Ga MW-based incandescent-type light source, with the emission wavelength centered at 527 nm; (c) EL emission from single AuNRs@ZnO:Ga MW-based incandescent-type light source, with the emission wavelength centered at 601 nm. (d) characteristics of single ZnO:Ga MW prepared via Au nanorods decoration (the extinction peak, 783 nm); (e) EL emission from single bare ZnO:Ga MW-based incandescent-type light source, with the emission wavelength centered at 518 nm; (f) EL emission from the single AuNRs@ZnO:Ga MW-based incandescent-type light source, with the emission wavelength centered around 780 nm. (g) characteristics of single ZnO:Ga MW via Au nanorods decoration (the extinction peak, 855 nm); (h) EL emission from the single bare ZnO:Ga MW-based incandescent-type light source, with the emission wavelength centered at 513 nm; (i) EL emission from the single AuNRs@ZnO:Ga MW-based incandescent-type light source, with the emission wavelength centered around 905 nm.
Consequently, incandescent-type light sources can be achieved from single semiconducting nano/MWs prepared via metal nanostructure decoration. It also can offer manageable, predictable, and ongoing requirements to construct electrically driven ultracompact optoelectronic devices. This experimental scheme can be extended to other wide bandgap semiconductors, such as GaN,
4. CONCLUSION
In summary, typical incandescent light sources were successfully constructed on account of individual AuNRs@ZnO:Ga MWs. By adjusting the aspect ratios of the Au nanorods, wavelength-tunable incandescent-type emissions can be achieved, with the dominating lighting wavelengths tuned from visible to near-infrared spectral bands. The intrinsic physical mechanisms being responsible for the emission modulation can be attributed to Au-plasmon-induced generation of nonequilibrium hot electrons that then are injected into the neighboring ZnO:Ga. Due to nonthermal distribution of the hot electrons with energy up to the plasmon frequency, the state-filling effect can be formed in the energy-level configuration. In particular, the experimental results represented further important progress to overcome the limitation of the bandgap of semiconductors involving electrically driving the generation and injection of energetically tunable, nonequilibrium distribution of hot electrons, plasmons mediated energy transfer, and the nonequilibrium hot-carrier-induced state-filling effect. Additionally, wavelength-tunable emission realized from individual MWs prepared with metal nanostructure decoration can overcome the traditional dilemma of semiconducting light sources with sophisticated fabrication technologies associated with p–n junctions, Schottky junctions, bandgap engineering, and so on. Consequently, the novel incandescent-type light sources may find potential applications in integrated optoelectronic devices, such as multicolor emission devices and electric spasers.
Article Outline
Zhipeng Sun, Mingming Jiang, Wangqi Mao, Caixia Kan, Chongxin Shan, Dezhen Shen. Nonequilibrium hot-electron-induced wavelength-tunable incandescent-type light sources[J]. Photonics Research, 2020, 8(1): 01000091.