1. Introduction

CdS is a typical II-VI group direct wide band gap material, which has been proved to be an excellent photoelectric material widely used in the field of nonlinear optics [1–3] and light-emitting diodes [4–6]. Lasing behaviour of semiconductor micro/nanostructures which can provide both active gain and optical waveguide cavities have intensively stimulated researchers’ interest in recent years due to their potential applications in intergraded optical circuit. Recently, Xiong’s group has realized the exciton-related photoluminescence and lasing in CdS nanobelts pumped by a pulsed Nd:YAG fourth harmonic (266 nm) laser [7] and Yang’s team has reported the NW lasers with controlling of emission wavelengths in combination with modes in individual bandgap-graded CdSSe NWs [8]. High lasing threshold of the nanoscale laser hinders its further application. For example: Wang’s group reported that the lasing of ZnO nanonails with the threshold is 22.4 MW/cm² [9]. Zhang’ group reported that the plasmonic lasing of nanocavity embedding in metalic nanoantenna array, and the experimental lasing threshold is about 270 MW/cm² [10]. Many studies focus on reducing the lasing threshold by different methods, such as using composition-symmetric nanowires, realized the threshold several times lower than that of composition-homogeneous wires [11]. With the assistance of Pt nanoparticle, Chen’s group decrease the lasing threshold from 5.6 MW/cm² to 4.28 MW/cm² for ZnO nanowires [12] and Li’s group reported that the surface plasma (SP) coupling also can be improved by using graphene, the graphene-ZnO hybrid nanowire laser represents a lower threshold [13]. Moreover, the total internal reflection (TIR) theory with the coating on the end facets of the microcavities was utilized to reduce the laser threshold [14]. Although these reports have a great impact on reducing the lasing threshold, these approaches rely on sophisticated fabrication techniques. Generally, the appropriate gas atmosphere is a simple and effective way to improve the performance of photoelectric devices such as source/drain (S/D) contact resistance of field-effect transistors [15–17], sensitivity of sensors and the electrical conductivity of electron devices [18–20]. In this paper, we explore the lasing threshold modulation effect of the tin-doped CdS micro/nanostructures in different gaseous environment and the threshold can be significantly decreased in Ar, N₂ and He environment.

In general, Fabry-Pérot (F-P) mode [21] and Whispering Gallery Mode (WGM) [22] are the common mode-types in nanowire laser. In principle, if the ends of the nanowire are smooth enough to confine more photon in the cavity by two cleavage plane, the nanowire can be defined as an F-P optical cavity. While, in the condition of local excitation in nanostructures, the formation of WGM will occur, which usually require a regular hexagonal cross-section exist. In this paper, we demonstrated that the structure dependent lasing mode of various CdS micro/nanostructures. And the time-resolved dynamic images for a complete luminescence process are obtained by using Digital Delay Generator(DDG)mode of ICCD, the whole delay time is matches with the carries lifetime. This method can provide a new means to observe the optical mode or waveguide, as well the lasing behaviour in the micro/nanostructures virtually.

2. Experiment

2.1. Materials synthesis

The multi-branched CdS comb-like microstructures were synthesized by a chemical vapor deposition (CVD) method. A mixture of CdS (Alfa Aesar, 99.999% purity powders) and SnO₂ powders (98% purity powders) [23], and the molar ratio is about 16:1, loaded in a ceramic boat, was placed in the center of a quartz tube, then the quartz tube was inserted into a horizontal tube furnace. Several Au-coated Si substrates were placed on both upstream and downstream of the source powders at a distance of 10 cm. Prior to heating, the quartz tube was purged with a mixture of Ar (90%) and H₂ (10%) for 30 minutes, with a constant gas flow rate of 30 sccm (sccm: standard cubic centimeters per minute). Then, the furnace was rapidly heated to 1000 °C at a heating rate of 100 °C min⁻¹, and the synthesis proceeded at this temperature for 60 minutes with a constant gas flow rate of 10 sccm. After the system was cooled down to room temperature, yellow products were found to deposit on the surface of Si substrate. For further study, the individual CdS comb-like structure was dispersed on the silicon substrate by high pure alcohol. Besides, the tin-doped CdS nanowires [24] were obtained by the same method, and the difference is that the gas flow with a constant rate of 30 sccm during the synthesis.

2.2. Setup of measurement

Scanning Electron Microscope (SEM, Carl Zeiss, SUPRA 55) combine with Energy Dispersive X-ray Spectroscopy (EDS) was used to study the morphology and elemental composition of CdS micro/nanostructure. Electron diffraction patterns were acquired by the Transmission Electron Microscope (TEM, FEI Tecnai F-20) for further investigation of CdS micro/nanostructures. The lasing behaviours of the various micro/nanostructures were investigated by a µ-photoluminescence (PL) system with an excimer pulse laser (CL5100, KrF, 248 nm, 6 Hz, 10 ns pulse width) as the excitation source (see Fig. S1, supporting information).The real-time images and time-resolved luminescence behaviours of the tin-doped CdS micro/nanostructures were obtained by using ICCD (Andor, DH334T, time resolution is about 2 ns) installed on the optical microscope (Olympus BX51M). Carriers dynamic behaviour was performed by a time-correlated single photon counting system (TCSPC) with a 405 nm picosecond pulsed laser as an excitation source, the time resolution was 40 ps.

3. Results and discussion

Figure 1(a) shows the SEM image of a typical CdS comb-like structure with periodically aligned branches, the branches are distributed evenly at one side of the trunk with a periodic distance of ~5 μm, diameter of around 4 μm and length of several hundred micrometers. The elemental mapping of CdS comb-like structure demonstrates that the main composition include Cd, S and Sn, as shown in Fig. 1(b). Figure 1(c) shows the morphology of a single CdS nanowire, which grows along [0001]. The electron diffraction pattern demonstrates that the nanowire is a typical wurtzite single crystal structure, the low-magnification SEM image demonstrates that the surface of the nanowire is smooth, the end is regular hexagon and the diameter is uniform. Figure 1(d) is the corresponding EDX elements mapping, which shows that the distribution of Cd, S and Sn is uniform.

 

Fig. 1 (a) SEM image of a typical CdS comb-like structure; (b) The elemental mapping of the comb-like microstructure; (c) SEM image of a single CdS nanowire, the inset is the low magnification image (left) and electron diffraction pattern of the nanowire (right); (d) The elemental mapping of the nanowire.

Download Full Size | PDF

Figure 2(a) shows the PL spectra of the multi-branched CdS comb-like microstructure under various power densities by using excimer pulse laser (248 nm). At low power density, a main band appears at 520 nm, which is attributed to the free exciton recombination. With the increase of the power density, the peak intensity of the main band becomes stronger, the FWHM of the band becomes narrow and a redshift of the peak position occurring, which means the excition-excition scattering and the formation of the electron-hole plasma (EHP) [25]. The inset of Fig. 2(a) demonstrates the PL image of the comb-like microstructure, UV light is focused on the truck part and the red circle marked in figure is the PL collection location. The dependences of PL intensity and FWHM (full width at half-maximum) of 520 nm emission on power densities shown in Fig. 2(b). With the power density increasing, the intensity increased nonlinearly and the threshold is around 5 MW/cm². When the power density reached to 6.83MW/cm², the FWHM becomes narrower and the minimum is 3.66 nm. It is hard to further compress the FWHM, due to no excellent oscillation cavity to be resonance cavity. Therefore, only amplified spontaneous emission can be occurred without narrow line lasing mode. In comparison, the power densities dependent emission of CdS nanowire is quite different from that of comb-like microstructures, as shown in Fig. 2(c). Even though at low power density, it is still a broad band at~513 nm (2.42 eV), which is attributed to the exciton spontaneous radiation. But with the power density further increasing, a new sideband (~521 nm) appeared at the low energy side of the mainband, the energy span between the mainband and the new sideband is equal to the energy of 1LO phonon. But the redshift can’t be attributed to exciton-phonon coupling simply. To consider the nanosecond pulse excitation covers more interactions involving exciton-exciton/electron-electron scattering [26]. Different from exciton-exciton scattering, some of the exciton will be dissociated at room temperature, so the free carrier usually is electron are scattered to a higher energy state rather than excitons. This process is called the exciton-electron (ex-el) or exciton hole (ex-h) composite, so the exciton-electron will become main mechanism at room temperature [27], and the characteristic spectra curve is similar to our reported lasing curve. Besides, we also consider the cavity effect together, such as WGM/F-P mode cavity. The different cavity length for F-P mode or WGM mode will make the peak shift of the emission due to the cavity confinement according to the formula$\Delta \lambda ={\lambda }^2/2nL$. If the gain band broad enough, then under high excitation together with the confinement effect, the multi-peak emission will occur generally. However, we also cannot see that, Therefore, we propose that the lasing mechanism is not due to cavity effect. The lasing behaviour of different diameter nanowire shown in Fig. S2. The power densities dependent FWHM in the inset of Fig. 2(c) shows that a superlinear behaviour occurred in the nanowire when the power density reached to 10.5 MW/cm². The FWHM of the sharp emission is 1.2 nm, which means the occurrence of highly-coherent resonance lasing mode.

 

Fig. 2 (a) The power densities dependent PL spectra of CdS comb-like structure, the inset is the real photoluminescence ICCD image of the comb-like structure, the excitation location was marked by the red circle; (b) The power densities dependent emission intensity and FWHM; (c) The evolution of photoluminescence spectrum of the tin-doped CdS nanowire as the power density increased from 5.23 MW/cm² to 10.5 MW/cm², the inset is the power densities dependent Intensity and FWHM. (d) The power densities dependent PL spectra of CdS nanobelt in atmospheric environment and the inset is real photoluminescence ICCD image.

Download Full Size | PDF

Moreover, the experiment is also performed in the CdS nanobelt, the power densities dependent PL spectra of CdS nanobelt under atmospheric environment as shown in Fig. 2(d). The spontaneous emission band was located at 515 nm (FWHM~25 nm) under a lower power density ~5.67 MW/cm². When the power density increased to 7.27 MW/cm², some sharp peaks upon spontaneous emission emerges out, indicating the transition from spontaneous emission to stimulated emission [28]. The intensity of the emissions obvious enhanced as the power density reached to 9.4 MW/cm². When the power density reached to 10.87 MW/cm², a strongest lasing emission peak can be distinguished in the emission spectrum, the FWHM is about 0.98 nm. The redshift of the main peak is due to the formation of EHP in the nanobelt under higher excitation. The sharp multipeak emission was realized with the high-quality oscillation with F-P mode resonance. The real time ICCD image is shown in the inset of Fig. 2(d), the width of the belt served as the feedback, the F-P lasing mode is formed in nanobelt cavity. The possible propagate route is marked in the image using black arrows. A typical multimode lasing mode occurs at around 529 nm with the FWHM ~0.98 nm, the$Q$factor is about 540 according to the equation $Q=\lambda /\delta \lambda$, where$\lambda$and$\delta \lambda$is the lasing wavelength and FWHM, respectively. Based on the standing waves equation$nL=k\frac{\lambda }{2}$, 5 lasing modes (505 nm, 513 nm, 521 nm, 529 nm and 537 nm) should be appeared between 500 to 540 nm. However, only three peaks at 521 nm, 529 nm and 537 nm are shown in Fig. 2(d) at higher power density, which is attributed to the mode competition in resonant cavity. Besides, modes spacing and width of the nanobelt is satisfied with the relationship of F-P cavity lasing mode:

Protecting gas is usually used to maintain the chemical stability and improve the performance of microdevice. Therefore, the lasing behaviour of the micro/nanosrtructures in different protecting gas is investigated. The integrated spectral emission intensity of CdS nanowire at various power densities in different gaseous environment as shown in Fig. 3(a). The ‘knee’ curves demonstrates that the lasing threshold is about 9.82 MW/cm², 8.25 MW/cm² and 7.22 MW/cm² in Ar, N₂ and He environment, the thresholds in various gaseous environment are lower than the threshold(10.5 MW/cm²) in air. One idea is the change of the surrounding refractive index when another kind of gas is used. But the refractive index of He gas is smallest ((1.000298) > Air (1.000292) > Ar (1.000281) > He (1.000035)), based on the law of total reflection, the lasing threshold in He environment should be highest, which is not consistent with our experimental results, so we exclude the influence of the surrounding refractive index. In addition, to consider some recently published paper, the gas molecules are easily ionized due to their lower ionization potentials than the excitation energy [31, 32]. Moreover, there exists some electrons in the micro/nanostructure surface under high power density, so the ionized gas molecules will be easily polarized by the electron on the around micro/nanostructure. Conversely, polarized gas molecules will conduce to confining parts of hot electron to make the decay process become slowly and induce population inversion in the nanowire. The smaller the gas molecules, the more polarized molecular accumulate around the nanowire, as the diagram shown in Fig. 3(b). Moreover, due to the presentence of polarized gas molecules, the electron scattering will be reduced in the nanowire, which induces the formation of stable population inversion in the nanowire. Therefore, the lasing threshold is lower in the He environment. The same experiment is also performed for the CdS nanobelt demonstrates that the lasing threshold of the nanobelt is also reduced with the decrease of gas molecular mass, which further confirm the theory that the smaller molecular mass, the lower lasing threshold. Similarly, for the amplified spontaneous emission of the CdS comb-like structures, it has the same effect. To better understand the lasing behaviour in different gaseous environment, the carrier dynamic behaviour of the nanowire was carried out using the TCSPC system with 2 mw picosecond diode laser as the excitation source, and the decay curves can be well fitted by the bi-exponential function. Figure 3(c) shows that the PL lifetime decay behaviour of the nanowire in air, Ar, N₂ and He environment, respectively, there is no significant change of the lifetime in different gaseous environment. The reason is that the gas is not easy to produce high-temperature plasma under the weak excitation, which differs from the change of lasing threshold under the strong excitation.

 

Fig. 3 (a) The output intensity dependence on power densities for the tin-doped CdS nanowire in air (black dot), Ar (red dot), N₂ (blue dot) and He (pink dot) environment, respectively; (b) Diagram of the nanowire in Ar, N₂ and He environment; (c) The carrier dynamic behaviour of the nanowire in different gaseous environment; (d) The output intensity dependence on power densities for the nanowire in vacuum and atmosphere condition.

Download Full Size | PDF

Despite the lasing threshold modulation effect in different protecting gas can be explained by the above reason, it cannot fully explain the lasing threshold is lager in the ambient air (the molecules mass of air is about 29) than in argon environment. Therefore, the lasing behaviour in vacuum and atmospheric condition were carried out, which gives a complete understanding and more information about the effect of lasing in different gaseous environment. The lasing threshold of CdS nanowire in vacuum is about 9.97 MW/cm², as shown in Fig. 3(d), it is smaller than that in atmospheric environment, the reason is attributed to electron depletion induced by adsorbed oxygen in oxygen-containing condition [33]. The decrease of the free carrier concentration after oxygen treatment mainly originates from an effective incorporation of oxygen atoms into the crystalline structure of the nanowire [34, 35], which lead to the lasing threshold become larger. However, the flowing gas reduces the concentration of oxygen, which avoid the depletion of electrons and increase the carrier concentration. In addition, new donor levels [36] will be introduced in protecting gaseous environment, which will lead to an additional recombination channel via these donor centers, and make the lasing threshold reduced. Therefore, the method of reducing the lasing threshold using different protective gas is a new approach to achieve lower threshold nanometer laser and promote the development of highly integrated photonic circuits.

In addition, time resolved ICCD images are used to investigate the dynamic waveguide and lasing behaviour in nanowire and more complex comb-like microstructure. The time resolved images of a complete photoluminescence process are obtained by using DDG mode of the installed ICCD (decay time is 2 ns). Meanwhile, the PL lifetime is also performed by the TCSPC system. The dynamics behaviour of the carrier and time resolved images of lasing emission of the nanowire as shown in Fig. 4(a). The decay time can be well fitted bi-exponentially, the short decay time ${\tau }_1$ is around 0.89 ns, which is ascribed to the delocalized carrier recombination process, and the long decay time${\tau }_2$~7.96 ns corresponds to the surface localized carriers [37]. The principle of TCSPC shows that the detecting probability of emitted photon is proportional to the fluorescence intensity, so it can be found that the time resolved ICCD images can well match with the PL lifetime. Similarly, the Fig. 4(b) exhibits the carrier dynamic behaviour and time resolved ICCD images of the CdS comb-like microstructure, the fitting result shows that the short decay time${\tau }_1$is around 0.92 ns and the long decay time${\tau }_2$is around 9.86 ns. It can be found that the long decay time of multi-branched microstructure is a little longer than that of nanowire, which originate from the smaller surface-to-volume ratio [38] in comb-like microstructure than that in nanowire. Therefore, this method provides a new way to explore the dynamic optical waveguide and lasing behaviour for different micro/nanostructures.

 

Fig. 4 (a) The time-resolved spectroscopy of the tin-doped CdS nanowire, the inset shows the PL image of nanowire under different delay time. (b) The time-resolved spectroscopy of CdS comb-like structure and the inset shows the PL image of multi-branched microstructures under different delay time.

Download Full Size | PDF

4. Conclusion

In summary, the lasing behaviour of various high-quality tin-doped CdS micro/nanostructures, including branched microstructures, nanowires and nanobelts, were demonstrated, which are synthesized by a well-controlled in situ source exchange CVD route. The lasing behaviour of nanowires, nanobelts is realized by exciton-electron scattering and the formation of EHP, respectively. However, the narrow-bandwidth lasing from the junction of comb-like microstructure is hard to realize compared to that in nanowire and nanobelt due to no good resonance cavities, and only amplified spontaneous emission was observed by the ICCD dynamic images of the photoluminescence. Moreover, the lasing behaviours of semiconductor micro/nanostructures were studied in different gaseous surrounding, and the lasing threshold of the nanowire reduces from 10.5 MW/cm² in air to 9.82 MW/cm², 8.25 MW/cm² and 7.22 MW/cm² in Ar, N₂ and He environment, respectively. It is attributed to the transient polarization of gas molecular. The results provide a general method to reduce the lasing threshold of various micro/nanostructures, which will have potential application in the realization of low threshold nanometer lasers and promote the development of highly integrated photonic circuits. In addition, the PL spectra and ICCD image, as well lifetime measurement prove the occurrence of lasing in nanowires and nanobelts with the pumping power increase, which provides a new method to explore the dynamic optical waveguide and lasing mode for the nanowires, nanobelts even the multi-branched microstructures.