1. Introduction

Colloidal quantum dots (CQDs) have long been candidates for optoelectronic applications due to their excellent luminescence property, facile solution processing to prepare large-area structure and narrow emission linewidth [1–3]. Specifically, CQDs are essential for lasing applications because of tunable wavelengths from UV to infrared, and the solution features make them widely applicable to any substrate [4–6]. However, it cannot be used as a cavity by itself due to the limitation of the size of CQDs. An effective way to control the spontaneous emission of light is to couple the luminous source with the optical microcavity. Effective coupling between cavity and CQDs is key factor to ensure output of lasing. For solution gain medium, it’s hard to get a balance between gain medium and cavity, too thick or too thin gain medium will cause light or gain loss. In sum, to obtain CQDs lasing, high photoluminescence quantum yield (PLQY) gain medium, high quality optical microcavity, and favourable coupling between them are indispensable.

Perovskite CQDs enable near-unity PLQY, tunable narrow emission and long lifetime, which are promising candidates for colloidal quantum dot lasing [7–11]. However, compared with bulk semiconductors, it is difficult for CQDs to be used in laser applications, because when they are excited by a single exciton, photon absorption and stimulated emission can reach a state of equilibrium, and no optical gain is achieved. Fortunately, in a cavity, the weak Purcell effect can greatly increase the laser modulation speed compared to the case in free space, and optical gain can be achieved by combining CQD with the cavity [12–14]. At present, the most common method for preparing CQD is by solution-based methods, such as inkjet printing, spin-coating and drop-casting [15–23]. Nevertheless, laser fabrication remains challenging due to the high demand for active optical amplification of CQD gain media and high quality small volume cavities.

The cavities like distributed Bragg reflector (DBR), photonic crystal microcavities used for perovskite CQDs lasers, which usually need expensive and complex etching or sputtering techniques [7–9]. Inkjet printing of CQD microcavities has significant advantages in terms of its ability to create high-quality, low-cost devices with improved performance characteristics [20–25]. However, the resolution of inkjet printing is limited by the nozzle size and it may require complex printing processes. Solution processed techniques like dip coating microcavities with perovskite CQDs provides a facile way to get lasing, however no lasing [24] or only random lasing was achieved [25].

Here we demonstrate a vertical tubular zinc oxide (VT-ZnO) with smooth surface and regular hexagonal section which can be used as a high quality microcavity for the CQD lasing. The VT-ZnO are well composited with CsPb(Br_0.5Cl_0.5)₃ CQDs, and the modulated PL and lasing under femtosecond excitation are achieved. The VT-ZnO/ CQDs composite shows lasing under 400 nm femtosecond excitation with a relatively low threshold of ∼ 46.9 µJ.cm^-2 and a Q factor of ∼ 2978 under excitation by a femtosecond (fs) laser.

We obtained blue CQDs lasing with low threshold of ∼ 46.9 uJ.cm^-2 and high Q factor of ∼ 2978 through combinating VT-ZnO with CsPb(Cl_0.5Br_0.5)₃ CQDs.

2. Results

We prepared VT-ZnO by chemical vapor deposition, which is a simple oxidation-reduction process. The synthesis of ZnO is done in a horizontal tubular furnace without catalyst and nitrogen. 0.06 g ZnO powder and 0.1 g carbon powder are mixed evenly and piled in a corner of a small quartz boat. A clean silicon wafer of 12 × 20 mm in size is used to cover the powder above. Then the quartz boat was driven to the centre of the tubular furnace. The temperature was raised to 1000 °C and maintained for 40 min, finally a large area of white crystal was formed after cooling in the air. Figure 1(a) is the scanning electron microscopic (SEM) images of VT-ZnO, the tubular structures with a regular hexagon section cover a large area on the silicon substrate. The cross-section radius of the tubular structures is around 1-5 µm, and the tube wall thickness varies. In addition, we can see that the outer surface of the micron tube is very smooth (Fig. S1), with a standard hexagonal cross section. Meanwhile, the inner surface of the tube is also very smooth, providing a foundation for subsequent high-quality optical microcavities. We further stimulated a single VT-ZnO through resonant fluorescence spectrum system with 325 nm He-Cd continuous laser. Figure 1(b) shows the PL spectrum of a single VT-ZnO, and the inset of Fig. 1(b) (Right) is the optical microscopic image of the VT-ZnO. Inset of Fig. 1(b) (Left) displays the calculated electromagnetic field distribution of such hexagonal WGM resonant optical field. The field-distribution is confined at the inner boundary of the VT-ZnO. The luminescence of VT-ZnO in the visible region is modulated by itself, and a series of regular resonance peaks occur, proving that the VT-ZnO can be used as a microcavity for ZnO/CQDs composite. By decreasing the amount of carbon powder to 0.06 g and reaction time to 30 min, the slant tubular zinc oxides (ST-ZnOs) were obtained, as seen in Fig. S2.

 

Fig. 1. (a) SEM image of VT-ZnO. (b) PL spectrum of a single VT-ZnO under 325 nm continuous excitation. Insets: Left is numerical simulation on the standing wave field distributions of pure ZnO and right is the optical microscopic image of a single VT-ZnO with 74X objective.

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In order to understand the growth mechanism of VT-ZnO, we further tracked how it formed by tracking the morphology of ZnO at different times during the synthesis process (Fig. 2). Firstly, Zn atoms and O atoms reacted chemically on the covered silicon, and a nanoscale ZnO was formed to act as nucleus, then several ZnO grew close to the nucleus (Fig. 2(a)). The size of nucleus and the surrounding ZnO determined the final size of VT-ZnO. Then the surrounding ZnO deposited to form a series of rod-shaped structures (Fig. 2(b)). As the reaction proceeded, the ambient atmosphere gradually changed from reducing atmosphere to oxidizing atmosphere, and rod-shaped ZnO gradually grew laterally, slowly becoming a wall (Fig. 2(c)), and till silicon zinc atoms were depleted in the nearby gas consumption, eventually forming the VT-ZnO samples (Fig. 2(d)).

 

Fig. 2. SEM images of ZnO at different times during the synthesis process to track the VT-ZnO formation.

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ZnO/CQDs composites were prepared by rinse dipping method. Firstly, 10 ml of diluted CsPb(Br_0.5Cl_0.5)₃ QDs were added into a 25 ml weighing bottle. A silicon wafer with tubular ZnO was inverted and fixed on a height adjustable sucker after vacuum pump opened. The wafer was immersed in QDs solution for 1 min and taken out, then washed by 10 ml of toluene. After being drying in the air, the above process was repeated till the ZnO changed from white to blue, eventually the ZnO/CQDs composites were obtained. Figure 3(a) is the SEM image of VT-ZnO after they composited with CsPb(Br_0.5Cl_0.5)₃ CQDs. We can see that the attached CsPb(Br_0.5Cl_0.5)₃ quantum dots are not very uniform. Figure 3(b) is the PL spectrum of single VT-ZnO/CsPb(Br_0.5Cl_0.5)₃ composite under 325 nm continuous excitation. The PL around 445 nm is from CsPb(Br_0.5Cl_0.5)₃ CQDs. Energy-dispersive X-ray spectroscopy (EDS) mapping (Fig. 3(c)-(h)) show O, Zn, Br, Cl, Cs, Pb elements uniformly distributed over the whole composite structure.

 

Fig. 3. (a) SEM image of a single VT-ZnO/CsPb(Cl_0.5Br_0.5)₃ CQDs composite. (b) PL spectrum of a VT-ZnO/CsPb(Cl_0.5Br_0.5)₃ CQDs composite. (c-h) EDS element mapping of VT-ZnO/CsPb(Cl_0.5Br_0.5)₃ CQDs composite showing CsPb(Cl_0.5Br_0.5)₃ QDs are well attached on VT-ZnO.

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In order to study the lasing performance on this VT-ZnO/CsPb(Cl_0.5Br_0.5)₃ CQDs composite, we further stimulated the structure using confocal fluorescence system equipped with 400 fs laser. With increasing pumping density, we observed lasing on a single VT-ZnO/ CQDs composite with radius of 2 µm (Fig. 4(a)). When pumping density increased to 46.9 µJ.cm^-2, a narrow emission peak appears at 450 nm. Above the threshold, the peak strength at 450 nm increases rapidly and is nonlinear to the pumping intensity. In this process, as the power increases, other resonant peaks occurred, when the pumping intensity increased to 63.4 µJ.cm^-2, the resonant peaks are at 448.97 nm, 448.1 nm, 447.5 nm, 447.1 nm. Figure 4(b) summarizes the output integrated intensity and FWHM as a function of pumping power. The output integrated intensity vs pump intensity indicated the transition from spontaneous emission to amplification. Therefore, based on the periodic peaks, the changes in power slope, and the reduction in FWHM, we can simply confirm the onset of lasing emissions within the VT-ZnO/CsPb(Cl_0.5Br_0.5)₃ CQDs composite system. The resonant peak at 448.13 nm was fitted by Lorentz function, and the half width is 0.151 nm, and the corresponding lasing quality factor is 2978 (Fig. 4(c)). Figure 4(d) shows the field-distribution of hexagonal WGM resonant optical field, the light was turned to be localized towards the outer boundary of the VT-ZnO/CsPb(Cl_0.5Br_0.5)₃ CQDs composite.

 

Fig. 4. (a) Pumping density dependent lasing from single VT-ZnO/CsPb(Cl_0.5Br_0.5)₃ CQDs composite. (b) The output integrated intensity (dots) and FWHM (squares) as a function of pumping density, and the lasing threshold is ∼ 46.9 µJ.cm^-2. (c) Lorentz fit of the lasing spectra showing Q factor ∼ 2978. (d) Numerical simulation on the standing wave field distributions of CsPb(Cl_0.5Br_0.5)₃ CQDs/ZnO composite microcavity.

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ST-ZnO and CdSe/ZnCdS CQDs are composited with the above rinse dipping method. Fig. S3a and Fig. S3b are the SEM images of ST-ZnO before and after they are composited with CdSe/ZnCdS CQDs. We can see that the attached CdSe/ZnCdS QDs are not very uniform. Fig. S3c is the PL spectrum of single ST-ZnO/ CdSe/ZnCdS composite under 325 nm continuous excitation. The PL around 525 nm is from ZnO and 640 nm is from CdSe/ZnCdS CQDs. An obvious cavity modulation was observed on ZnO peak, but no modulation on CQDs peak, indicating no coupling between ST-ZnO/ CdSe/ZnCdS composite. VT-ZnO and CdSe/ZnCdS CQDs are also composited with the above rinse dipping method. Fig. S4 shows the PL spectrum of ZnO/CQDs composite under 325 nm continuous excitation, and the inset is the SEM image of a single VT-ZnO/ CdSe/ZnCdS composite, which shows that CQDs are attached uniformly. The PL peaks at around 510 nm and 640 nm are from VT-ZnO and CdSe/ZnCdS CQDs, respectively. It can be seen that the PL of CQDs is obviously modulated by VT-ZnO, a series of small resonant peaks appear, and the width at half height of the resonant peaks is ∼ 5 nm. This indicates that VT-ZnO shows obvious coupling with CQDs.

3. Conclusions

In sum, we report a new method to grow high quality VT-ZnO as optical microcavity and provide a way to complex high quantum yield perovskite CQDs with these ZnO. Lasing was obtained with threshold of ∼ 46.9 µJ.cm^-2 and the corresponding quality factor is ∼ 2978. In order to obtain the quantum-dot microcavity composite laser, we need the gain medium with high luminous efficiency and the microcavity with good quality, and the coupling between them is also an important factor.