Micro-pump fluidic strategy for fabricating perovskite microwire array-based devices on semiconductor platforms and method

A method for making ion-crystal semiconductor material based micro- and/or nanowires, MNWs, embedded into a semiconductor substrate, includes forming a structure into the semiconductor substrate, wherein the structure has each of a width and a depth less than 10 μm; pumping an ion-crystal semiconductor material as an ion solution into the structure, wherein the pumping is achieved exclusively due to capillary forces; flowing the ion solution through the structure to fill the structure; crystallizing the ion-crystal semiconductor material inside the structure to form the MNWs; and adding electrodes to ends of the MNWs.

BACKGROUND

Technical Field

Embodiments of the subject matter disclosed herein generally relate to a system and method for forming a perovskite and other liquid-processed material microwire and nanowire array-based device embedded into a semiconductor platform, and more particularly, to using a micro-pump fluidic strategy for forming the microwire array-based device.

Discussion of the Background

In the past few decades, micro-/nanofluidic technologies have played an important role in biological and medical diagnostics and biochemical research, leading to cutting-edge technologies such as lab-on-a-chip (LOC) devices. These advanced processes can be used to precisely manipulate both the fluid flow (e.g., solvents) and mass transport of small molecules (solutes) at a micro-/nanoscale in semiconductor platforms, thereby facilitating the manufacture of millions of microchannels, each measuring mere micrometers, on a single chip. However, as the material required for the application of this technology must be in fluid form, limited progress has been made in this field over the years, necessitating further work on advancing the control method to make it applicable to semiconductor materials.

Recently, lead-halide-based perovskites (MaPbX3and CsPbX3), as a new type of ion-crystal semiconductor material, have attracted considerable research interest because they can be synthesized by low-cost solution processing at room temperature and because of their unique characteristics, such as high-quantum yield (up to 90%), tunable emission spectra over the entire visible range with narrow linewidth, suppressed photoluminescence (PL) blinking, high carrier mobility, and large diffusion length [1-5]. Thus, perovskite materials have emerged as suitable candidates for a wide range of electronic, optoelectronic, and photovoltaic applications.

Perovskite can be integrated into other semiconductor devices to obtain different functionalities, such as adjusting the charge carrier separation, enhancing light capture, and optimizing the optical parameters of these devices. Integrating patterned nanoscale perovskite structures into semiconductor-based miniaturized devices is especially used for fabricating lasers, solar cells, and photodetectors (PDs) due to their characteristics—in particular, their high surface-to-volume ratio, which results in high sensitivity, rapid response time, and low power consumption. For example, functionalizing Si with perovskite has been demonstrated for developing tandem solar cell devices, using perovskite film structure. Patterned structures based on a wide range of traditional materials, such as Si, conducting polymers, metal oxides, and other semiconductors have already been obtained; nonetheless, it is still challenging to obtain patterned ion semiconductor crystals such as perovskite using the traditional lithography technology to be integrated with semiconductor devices. Note that an ion-crystal is understood herein to be formed of cations and anions with very large electronegativity differences, such as alkali metals and halides, columns I and VII of the periodic table, respectively.

When perovskites were subjected to conventional solution-based processes aimed at functionalizing them in nanofabricated devices, their dewetting behavior was very difficult to control using drop-casting, spin-coating, or inkjet printing [6]. For example, previously proposed methods for synthesizing patterned polymer and other solution-processed materials, such as the liquid knife method [7], the capillary-bridge method [8, 9], the nano-channel-assisted method [10], the wettability surface control method [11, 12], and the microchannel-confined crystallization strategy [13] are costly and require complex fabrication processes, which necessitate the use of additional tools. Moreover, in these methods, the nanoscale resolution is not optimized, and the excess perovskite remaining after the device fabrication cannot be recycled.

Thus, there is still a need for a cost-effective, simple, and feasible approach that can produce waste-free, one-dimensional (1D) microwire (MW) arrays of perovskite with nanometer dimensions, which are embedded in a semiconductor platform.

BRIEF SUMMARY OF THE INVENTION

According to an embodiment, there is a method for making ion-crystal semiconductor material based micro- and/or nanowires, MNWs, embedded into a semiconductor substrate. The method includes forming a structure into the semiconductor substrate, wherein the structure has each of a width and a depth less than 10 μm; pumping an ion-crystal semiconductor material as an ion solution into the structure, wherein the pumping is achieved exclusively due to capillary forces; flowing the ion solution through the structure to fill the structure; crystallizing the ion-crystal semiconductor material inside the structure to form the MNWs; and adding electrodes to ends of the MNWs.

According to another embodiment, there is a lab-on-chip device that includes a semiconductor substrate, a power source integrated into the semiconductor substrate, a sensor integrated into the semiconductor substrate, and a processor integrated into the semiconductor substrate. The processor is configured to receive a measurement from the sensor and the power source is configured to supply electrical power to the sensor and the processor. Each of the power source, the sensor, and the processor includes plural micro- and/or nanowires, MNWs, formed inside corresponding plural micro-channels formed to the semiconductor substrate, and each of a width and a depth of each micro-channel of the plural micro-channels is less than 10 μm.

In yet another embodiment, there is a method for making a liquid-processed material based micro- and/or nanowires, MNWs, embedded into micro-channels formed on a semiconductor substrate. The method includes forming a structure on the semiconductor substrate, wherein the structure has each of a width and a depth less than 10 μm; pumping a liquid-processed material as an ion solution into the structure, wherein the pumping is achieved exclusively due to capillary forces; flowing the ion solution through the structure to fill the structure; crystallizing the liquid-processed material inside the structure to form the MNWs; and adding electrodes to ends of the MNWs.

DETAILED DESCRIPTION OF THE INVENTION

The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to a specific perovskite material that is embedded as microwires in a semiconductor structure. However, the embodiments to be discussed next are not limited to a single perovskite material, or to a semiconductor structure, but may be applied to other ion-crystal materials or to any liquid based material that can be transformed in a solid material. In one application, a different material is deposited on the semiconductor substrate and the micro- or nanochannels are formed in this material, which may be, for example, a metal-oxide frame.

According to an embodiment, a micro-/nano-fluidic method relies on a micro-pump auxiliary strategy to obtain well-aligned ion-crystals (e.g., perovskite) micro- and/or nano-wires (MNWs) embedded inside patterned semiconductor (e.g., Si) micro-channels (thousands of microchannels). These micro-channels are fabricated, in one application, by cost-effective, mask-free, laser interference lithography (LIL). The proposed method produces high-resolution (in hundreds of nanometers) MWs, while ensuring uniform control of the crystal size, with the potential for application in integrated circuits in a chip. In one embodiment, which is discussed later, a high-performance photo-detector (PD) based on CsPbBr3MNWs integrated into a patterned SiO2/Si platform is disclosed, confirming that it can be extended to large-scale microwire applications, as it is inexpensive, highly efficient, and flexible.

FIG.1Ashows a micro/nano-pumping system100for fabricating MWs of solution-processed ion-crystal semiconductor material (e.g., perovskite) that subsequently can be embedded in corresponding micro-channels of a patterned semiconductor device. The system100includes a microwires embedded platform102, a micro-pump140, and a container122that holds an ion solution120, which is later crystalized to form the MWs132. The microwires embedded platform102(herein “the platform”) includes a patterned substrate (for example, Si substrate)110that has plural micro-channels112extending along a longitudinal axis X of the substrate. Although the name of the micro-channels112appear to suggest that their size is in the micrometer range, these channels can be as small as possible, for example, in the nanometer range. The micro-channels112may be parallel to each other and also they may be formed as a straight line or have different shapes and/or profiles. The ion solution120, which includes the perovskite material130and a solvent, is stored in the container122. The micro-pump140fluidly connects the container122to the platform102and moves through capillarity the ion solution120into the plural micro-channels112. In this embodiment, the micro-pump140has a substrate142(for example, made out of glass or plastic, i.e., a solid material) to which a paper filter144is attached. The paper filter is usually soft, i.e., bendable and cannot hold its shape without the help of the substrate142. The paper filter144has a myriad of capillaries that feed into the ion solution120. Due exclusively to the capillarity effect, the ion solution120is moved through the paper filter144into the micro-channels112, which means that the length L and the width W of the paper filter are selected depending on the ion crystal semiconductor material used to make sure that the ion solution reaches the micro-channels in an amount that at least replaces an evaporation of a solvent of the ion solution, as discussed later. In one embodiment, a length of the micro-channels may be in the range of micrometers to centimeters, while the width is 10 μm or less, or 1 μm or less, and the height is 10 μm or less or 2 μm or less. Note that other materials may be used instead of the paper filter144as long as they provide capillarity force to move the ion solution between the container122and the micro-channels112.

FIG.1Bshows a more complex platform102that has not only the parallel micro-channels112, but also a non-straight line micro-channels113, and additional parallel micro-channels112-2that are separated from the original micro-channels112by a central reservoir124. Further, the platform102may have an inlet reservoir126where the ion solution120received from the micro-pump140is first received. Note that the micro-pump140touches a proximal end110A of the substrate110. The ion solution120is then distributed from the inlet reservoir126to the parallel micro-channels112and to the non-straight line micro-channel113. Note that the micro-channel113may take any shape. Also, the figure shows for simplicity a single micro-channel113, but more such micro-channels may be formed. In one application, all these micro-channels feed into the central reservoir124. The central reservoir may be located anywhere on the substrate110. The additional micro-channels112-2originate from the central reservoir124and may extend to the distal end110B of the substrate. With this structure, more elaborate components may be formed, for example, a solar cell, a laser, a transistor, an integrated circuit, etc.

The fabrication of the patterned substrate110with periodic line gratings or micro-channels112is now discussed with regard toFIG.2, which schematically illustrates the LIL process. Note that the perovskite material130is very brittle and manipulating this material is very difficult. However, with the micro-channels formed in the semiconductor substrate110, the MWs130are directly formed in the mold provided by the micro-channels, and thus, the MWs do not break. LIL is a facile, inexpensive, rapid, and mask-free patterning technique for fabricating periodic and uniform micro-/nanopatterns (e.g., line, dot, hole arrays) across a relatively large substrate area. The LIL performed in this embodiment includes three steps. During the first step, the Si substrate110is covered by a photoresist202. The photoresist was prepared by mixing AZ2020negative photoresist and AZ1500thinner (at the 1:0.8 ratio) by string for 48 hours. 200 nm of the prepared photoresist was spin coated (at 4000 rpm for 45 s) on an O2plasma-treated (30 sccm for 1 min) Si wafer110. After an 80 s soft-bake process at 100° C., the as-coated wafer was transferred to the LIL system for exposure to two interfering laser beams204and206, which were generated with a 26 mW 325 nm He—Cd laser serving as the light source. A beam expander was utilized to generate a larger light spot with a diameter of 12 cm. The line gratings and the period width were determined by the exposure time and the angle of the stage (20 s˜25 s was adopted in this embodiment, but other times may be used). The period could be adjusted, while remaining in the 160-2000 nm range. After conducting a further 180 s post-bake at 115° C. to ensure photoresist stability, the AZ762 MIF developer was applied for ˜40 s to obtain the desired design pattern208shown in the figure. All aforementioned steps were performed in darkroom to avoid ambient light from affecting the photoresist exposure.

Next, for the etching process, a deep-reactive ion Si etch process210was applied to create the desired pattern112on the Si wafer110. This etching process used, in one application, 5 s develop step of 100 sccm C4F8, 10 sccm SF6and 7 s etch step of 5 sccm C4F8, and 100 sccm SF6alert flow into the etching chamber. Normally, the alert was conducted in 10˜20 cycle in the case of 200 nm thickness photoresist. Acetone washing212to remove the residuum of photoresist was then performed. Thus, a deeper and stable periodic Si microchannel112that can withstand application of most organic solvents is obtained without the need for the unstable conventional photoresist patterning process. Finally, to fabricate an insulating oxide layer114on the Si patterned substrate110, the periodically patterned Si substrate was further subjected to a standard thermal oxidization process214to create a thin SiO2layer114on the patterned Si microchannel112's, as shown inFIG.2. In one application, the SiO2layer has a 300 nm thickness and it was deposited on the Si patterned channels/ridges using a standard wet rapid thermal oxidation.

The LIL technique allows the fabrication of different periodic patterns (e.g., holes, pillars, parallel line gratings), by modulating various lithographic parameters, such as interference light intensity, angle, exposure time, and development duration. To obtain the parallel line grating-patterned matrix shown inFIG.2, a single exposure was performed. However, to obtain an array of periodic hole (or pillar) patterns, a second exposure needs to be performed. The patterned Si substrate110made in this embodiment is comprised of a microchannel array that is characterized by about −1 μm channel/ridge periodic pairs with a depth of 1-2 μm. Moreover, the obtained channel depth and width are uniform. A liquid droplet of the ion solution120exhibits an anisotropic shape on the parallel line grating-patterned Si surface, demonstrating a guide function for the liquid flow process.

WhileFIG.2illustrates the formation of the micro-channels112into the Si based substrate110, those skilled in the art would understand that there are other materials and/or methods for forming the micro-channels112. For example, in one application, it is possible to grow a metal-organic frame (MOF) and pattern it on the semiconductor substrate110to obtain the micro-channels112. The patterned MOF can play the role of the transport liquid instead of the Si channels. Thus, it is possible to site-selective growth of metal-organic frameworks using an interfacial growth approach combined with VUV photolithography to achieve the micro-channels112.

Next, the process of incorporating the perovskite material130into the microchannels112is discussed. The process includes two stages. The first stage is the injection of the ion solution120into the proximal ends112A of the empty micro-channels112through the filter paper144, while the second stage shows the post-“evaporation-injection-balance” process, when the perovskite MWs132are formed, starting at the distal ends112B of the micro-channels112.

In this embodiment, the ion solution120includes CsBr and PbBr2dissolved in dimethyl sulfoxide (DMSO) solvent and this composition was placed in the container122inFIG.1A, which serves as a “solution source.” Next, the filter paper144was placed on a piece of a glass slide142, to act as the micro-pump140to transmit the ion solution120from the solvent source122(using exclusively the surface tension force) into the proximal end112A of the parallel micro-channels112of the substrate110. One side/end140A (seeFIG.1A) of the filter paper/glass pump140was dipped in the perovskite source solution122and the other side/end140B was attached to the first ends112A of the micro-channels112of the substrate110, as shown inFIG.1A. In several minutes, the filter paper144became fully wet and then the DMSO solution injection process (including Cs, Pb, and Br ions) into the micro-channels112started by relying solely on the capillary force.

Due to the high-energy surface of the plasma etched area inside the micro-channels112(with more dangling bonds of Si along the etched surface), the liquid solution120could thermodynamically adhere to the micro-channel surface. Therefore, the ion solution120was fully contained along the micro-channels112and had the same level as the ridge height without dips or overflow, as shown inFIG.3A. Note that the region300inFIG.3Ahas all the micro-channels112filled with the ion solution120while the region310has the micro-channels empty. After the DMSO solvent evaporated from the ion solution120in the micro-channels112, as schematically illustrated by arrows116inFIG.1A, the remaining Cs+, Pb2+, and Br ions in the solvent were clustered and nucleated and crystallized inside the channels112, forming the perovskite MWs132, on the patterned Si substrate110, as shown inFIGS.1A and3B. The energy-dispersive X-ray spectroscopy (EDX) elemental maps generated by the inventors for this configuration demonstrates that the elemental compositions (e.g., Cs, Pb, Br) are almost uniformly distributed along the entire periodic array of perovskite MWs. Note that the width of the micro-channels112controls the MW132's width.

The previous embodiments illustrated a growth process of perovskite MWs132within the Si micro-channels112. During the growth process, it was observed that after the micro-channels112are filled with the perovskite solution120, the solution level starts decreasing due to the evaporation116. The inventors also observed that driven by the capillary forces, as more ion solution120is pumped from the solvent source122, this new ion solution120pushes the as-evaporated solution forward to the distal end112B of the micro-channel112, which is distal from the pump140, to compensate for the lost volume. As a result, the ion concentration distribution in the solution gradually increases from the proximal end112A at which the solution is pumped into the micro-channel toward the distal end112B. Thus, the perovskite MW132crystallization progresses sequentially in the reverse direction, i.e., from the distal end112B to the proximal end112A, as indicated by the arrow410inFIG.4, when compared to the direction412of the solute flow. The line414is the ion concentration axis, indicating a high ion concentration value toward the distal end112B and a low ion concentration value toward the proximal end112A of the micro-channel112. This results in the formation of a perovskite MW132confined by two adjacent Si ridges402and404. Note that the dimension of the perovskite MW132depends on the micro-channel112's dimension. Thus, this evaporation technique can be used to form the perovskite MWs132as the LIL method was successfully used for producing the nano-channels112.

To gain insight into the ideal MW growth shown inFIG.4in a micro-channel112-1, a computer simulation of the flow character and ion distribution was performed using the COMSOL software. To simplify the analysis, while retaining the channel width and height (0.72 and 1.5 mm, respectively in this embodiment) used in the experimental setup, the channel length was shortened to 20 mm. At the same time, the amount of the injected solution was regarded as equal to the evaporation amount. The injection direction was from the proximal end112A of the micro-channel112, while the evaporation occurred at its top surface, as shown by arrow416inFIG.4. The DMSO solution evaporation rate was found to be about 0.8 pg/s/mm2at the “evaporation-injection-balance” stage. To show the micro-pump process more clearly, it was assumed that the ions were transported with the solution and had a low diffusion propensity (depending on their density). The initial concentration was set to 225 mmol/L, in line with the experimental value.

It was found from the software simulations that the solution velocity exhibits a U shape, whereby the velocity at the center is much greater than that near the channel walls due to the boundary effect. Moreover, the velocity decreases from 16 mm/s at the proximal end112A of the channel112to about 0 at its distal end112B. At the start of the process (i.e., at t=0 s), the ion concentration is 225 mmol/L and is evenly distributed across the channel112. As the time passes, the evaporation induces an increase in the ion concentration. Thus, at t=30 s, the ion concentration in the channel112has a distribution from 225 mmol/L at the proximal end112A of the channel112(L=0 mm) to 246 mmol/L at its distal end112B (L=20 mm), as shown inFIG.5. The higher ion concentration in the channel112indicates that the perovskite crystallization starts from the channel's distal end112B. These simulation results support the hypothesis regarding the ideal growth conditions shown inFIG.4.

Defects such as discontinuities420and overflow defects422(the most common defects) as shown inFIG.4can appear in the MWs132. Scanning electron microscopy (SEM) images confirm these different types of defects, including nucleation424, discontinuities, uneven distribution in the horizontal direction, and overflow. These defects prevent the formation of periodic MWs across a large area of the patterned substrate110. To avoid the emergence of these defects, it is desired to eliminate random nucleation and properly let the solution120flow to the end of the micro-channels112. Although the LIL allowed the fabrication of the micro-channels112with a high length-to-width ratio, accumulation of the perovskite solution120can still occur in such a long channel, leading to the aforementioned defects.

To address these issues, the micro-pump process illustrated inFIG.1Awas modified in this embodiment by using another strategy. The substrate110was scratched using a diamond knife, along a direction perpendicular to the Si microchannel arrays112(i.e., the liquid flow direction), to form a trench610, as illustrated inFIG.6. This scratch was highly effective in releasing the pump force, i.e., offering the ion solution120an escape path, and ensuring an even and homogeneous redistribution of the perovskite solution120inside the micro-channels112. To obtain uniform periodic dimensions of MWs, this configuration was further optimized. For example, based on the scratch process, focused ion beam (FIB) lithography was used to delicately form plural lines620on the patterned substrate110, which are perpendicular to the microchannels112, as shown inFIG.7. By fabricating these lines620, an easy-flow and growth strategy was obtained by eliminating discontinuities or overflow defects, resulting in a periodic perovskite MW array112with about 100 mm length at an area of several centimeters (i.e., 500-nm width was achieved for a single MW). As the MW dimensions depend on the micro-channel's dimensions using this micro-pump process, the MW height is determined by the micro-channel's depth, which is limited by the photoresist thickness. However, theoretically, the MW length is expected to be flexible (up to several centimeters), but may be controlled by the defect presence.

Another method to further improve the quality of the perovskite/Si platform102for use in practical devices is now discussed with regard toFIG.8. To further optimize the proposed micro-pump strategy without using the previously described scratching configuration shown inFIG.6, an in situ accelerate evaporation device800can be placed at and above the boundary810between the crystallized material132and the ion solution120, as shown inFIG.8. The device800may include a housing802, in which a propeller804is configured to rotate due to a motor806. An air jet808is then directed by the housing802toward the boundary810, to enhance the evaporation process. In one embodiment, either the device800or the substrate110is configured to move along arrows812, relative to each other, so that the air jet808moves with the boundary810. In this case, the MW crystallization rate can be accelerated and the upstream pump force pressure can be released. The release of the pump force would induce fluid flow acceleration820, which could prevent nucleation from occurring in the middle of the channel, thereby eliminating MW discontinuities. This strategy can be adapted to quickly transport the ion solution120inside the Si micro-channels112and to reduce nucleation, while quick evaporation would release both the pump force and the trapping force. As shown inFIG.4, the solution stream is separated well by the Si ridges402,404, indicating that the MWs132formed in the micro-channels112are free of discontinuities and overflow defects. It is assumed that, if the material volume did not change during the crystallization process, which would be the case if the phase changed from liquid (e.g., water) to solid (e.g., ice), no defects would exist.

In contrast to the traditional fabrication methods, the novel micro-pump fluidic method discussed herein is significantly cost-effective, simple, and feasible as it requires only filter papers between the source122and the patterned platform110. Traditional methods reported in the literature used complicated fabrication processes for fabricating 1D MWs, and the resulted microwires have micrometer resolutions. For example, in these studies, photolithography was used to fabricate periodically aligned SU-8 photoresist stripes on the SiO2/Si substrate, which acted as the template for the subsequently aligned growth of MWs and via traditional blade-coating/dip-coating methods.

In addition, the novel micro-pump approach benefits from self-assembly and auto- or self-growth technology based on the capillary force effect in micro-channels (carried out under the ambient conditions) without the need for the complexity of the photoresist and lithography methods and can be applied to any patterned solid platform with nanometer dimensions. In addition, the novel approach discussed herein does not generate any undesirable by-products; it is a zero-waste and contamination-free process as the filter paper could be easily cleaned and thus recycled by dipping it into the DMSO solution. Furthermore, this micro-pump process prevents the contamination of raw materials. Moreover, the LIL process used to fabricate the Si micro-channels is based on a cost-effective, simple, mask-free patterning technique, whereas traditional lithography is costly and complicated. Owing to its simplicity, the novel process disclosed herein does not require a mask aligner for fabricating periodically aligned micro-channels. In addition, transferring the MWs embedded in the Si micro-channels into another substrate is possible.

To determine the structural and optical properties of the perovskite MWs132embedded in the Si microchannel substrate110, the inventors have carried out a transmission electron microscopy (TEM) analysis to ascertain the MW size and structural homogeneity. Thus, a cross-section of the perovskite MW/Si periodic array was prepared via the FIB technique. The examined uniform array comprised 14 MWs well separated by Si ridges402/404. Each MW in this embodiment has a height of 1.5 mm and a width of 0.72 mm and is fully embedded in the Si channel with no interspaces or defects, further confirming the effectiveness of our micro-pump microfluidic method.

The high-resolution TEM (HR-TEM) and the fast Fourier transform (FFT) results, which were measured for the perovskite MW/Si lamella prepared by SEM-FIB, suggest that the perovskite MW exhibits a major cubic crystalline structure with slight segregations. The X-ray diffraction (XRD) was performed to further confirm the crystalline quality of the perovskite MWs embedded in the micro-channel, as shown inFIG.9. The peaks900match the XRD peaks of the typical cubic CsPbBr3structure. This result confirms that the main chemical compound of these MWs132is the CsPbBr3, as observed in the HR-TEM images. The minor peaks910, correspond to the CsBr segregation formed due to its low solubility in the DMSO solvent. As reported in the pertinent literature, the other obscure peaks can be associated with zero-dimensional perovskite Cs4PbBr6. These findings are in line with the HR-TEM results above, identifying the origin of the minor observed segregations. The existence of segregation and chemical phase in perovskite is determined by modifying the molar ratio of PbBr2and CsBr in the DMSO solvent that was suggested to be desired for improving the purity of the chemical phase.

To investigate the optical quality of the perovskite MWs132embedded in the micro-channels112, PL and time-resolved PL (TRPL) measurements were carried out at room temperature.FIG.10shows the PL spectrum1000of the perovskite MWs132, indicating an intense emission peak centered at 524.5 nm with a full width at half-maximum (FWHM) of 26.2 nm. To study the origin of this peak, the PL excitation (PLE) spectrum1010at 492.4 nm shows a Stokes shift with respect to the PL peak, which is in line with the reported work for CsPbBr3nanocrystals. The lifetime decay curve1100is shown inFIG.11. The lifetime components were obtained by fitting the experimental data1100to the biexponential lifetime decay model1110. The calculated fast (15.1 ns) and slow (about 43.6 ns) lifetime components indicate different recombination centers related to trap-assisted and free charge carrier transition decay, respectively. The total lifetime of 38.4 ns is 8 times longer than that of pure CsPbBr3nanocrystals previously reported, indicating a higher radiative recombination rate in the perovskite MW array. Thus, these findings (including TEM and XRD results) indicate that the high optical and structural qualities of the perovskite MWs132confined in Si micro-channels112may comprise minor Cs4PbBr6nanocrystals (segregations) resembling a CsPbBr3matrix.

The novel MW/Si platform102can be used for making an optoelectronic device. For example, a PD device1200based on perovskite MWs132confined in the Si micro-channels112(acting as a semiconductor platform102) was fabricated. A 300-nm-thick SiO2layer114was grown on the patterned Si micro-channels112via thermal oxidation (as show inFIG.2), to ensure that no response is obtained from the Si substrate110, while collecting responses only from the perovskite MWs132. Then, the micro-pump method illustrated inFIG.1Awas used to form the MWs132confined in the SiO2/Si micro-channels112. Next, for metal contacts, Ti and Au interdigitated electrodes (IDEs)1210and1212of 100-nm thickness were deposited on top of the perovskite MWs132by magnetron sputtering assisted by a shadow mask, as shown inFIG.12.FIG.12shows that 27 individual perovskite MWs132were created, each of which was in contact with the two electrodes1210and1212. In one application, the interdigitated contact electrodes included four close parallel branches extending from two separate trunks. The channel length between the branches and the branch width was 30 and 950 mm, respectively. Note that line1220indicates the micro-channels112that were not filed with the perovskite material130while arrow1222show a completely filled micro-channel. The obtained PD1200was then tested to determine the transient curves at various bias voltages and the I-V performance.

More specifically,FIG.13shows transient on/off cycles of photocurrents measured at various voltages (0.01, 0.1, and 1 V), as well as 0 V, based on a typical ohmic contact shown in the I-V curve (current-voltage characteristic curve) illustrated inFIG.14. The results indicate that the PD device1200can work as a self-powered device. A 0.14-mW/cm2white light-emitting diode (LED) served as a light source. The resulting device1200was stable and was capable of reversible and rapid switching between the dark and the illuminated states. Exposure involving140on/off cycles resulted in a stable responsivity of about 0.96 NW. However, the high dark current is limited under high bias and needs to be optimized. If the dark current effect is ignored, then the photocurrent is as high as 1 mA under 1 V voltage bias, which is higher than the value previously reported for nanocrystal perovskite-based PDs. The high dark current at high bias can be attributed to two reasons. First, the interfacial defects between the perovskite material130and the SiO2114increase the dark current. Second, the inclusion of Cs4PbBr6in CsPbBr3can reduce the photo-response, as the Cs4PbBr6has a lower photo-response when compared to the CsPbBr3.

Furthermore, the inventors performed a 10,000 Hz communication experiment, but no response was obtained, suggesting a 0.1 μs to less than 80 ms response time range, which is comparable to that of reported perovskite-based PDs. In general, the PD1200's performance (including responsivity and response values) is higher than those in several reported works. In particular, the responsivity of the PD device1200is higher than that of previously reported self-powered, PD-based on perovskite. Thus, the novel microfluidic strategy shown inFIGS.1A and1Bdemonstrated high-performance MW-based PDs compared to previously reported PDs based on perovskite [14, 15] or other materials under low applied voltage.

The above embodiments disclose a cost-effective and superior micro-pump (microfluidic) strategy for fabricating well-aligned, parallel, perovskite MW arrays confined in a Si patterned (microchannels) platform. One possible advantage of the proposed strategy is its high-resolution and zero waste and chemical pollution, making it feasible for large-scale perovskite-based applications. Advanced optical and structural characterizations revealed the good quality of CsPbBr3MWs with minor inclusions of Cs4PbBr6nanocrystals. Moreover, the successful fabrication of a highly sensitive self-powered PD based on these micro-pump-assisted perovskite MWs confirmed that this method can assist in producing 1D nanostructures whose characteristics can be adjusted by modifying the semiconductor microchannel dimensions, such as photovoltaic cells, high-density microcircuits, field-effect transistors, biosensors, waveguides, and mersisters. The simplicity and cost-effectiveness of this strategy (owing to room-temperature processing without the need for expensive facilities) would ensure its scalability.

In one embodiment, it is possible to use the method illustrated inFIGS.1A and1Band make a lab-on-chip device1500, as illustrated inFIG.15. The lab-on-chip device1500includes a single substrate1502on which various elements are embedded. For example, the PD device1200can be formed directly on the substrate1502. Another sensor1510may be formed directly on the substrate1502, and the sensor1510may be made with the method shown inFIGS.1A and1B, but may be used as another type of sensor. Any other sensor may be implemented on the substrate1502, for example, a transistor, light-emitting diode, or a solar cell, a laser device, etc. Each or any of these devices may be manufactured on the common substrate with the method illustrated inFIGS.1A and1B. A battery1520may also be manufactured directly on the substrate with the method ofFIGS.1A and1Band connected to the various sensors to provide electrical energy. For example, the battery1520may be a solar cell that has perovskite MWs132formed in the micro-channels112of the substrate1502. A processor1530that includes at least a transistor may also be formed directly on the substrate1502, either by the existing lithographic methods or by using the procedure illustrated inFIGS.1A and1B. One skilled in the art would understand that for a Si substrate, any known semiconductor device may be formed on the substrate and integrated with the perovskite MW based device1200. Also, those skilled in the art would understand that other ion-crystal semiconductor materials may be used instead of the perovskite material to form the MW of the device1200. In one application, the processor1530receives a measurement from the device1200and the battery (solar cell)1520supplies electrical energy to both the device1200and the processor1530.

A method for making any perovskite MWs based semiconductor device is now discussed with regard toFIG.16. The method includes a step1600of forming one or more structures112in a semiconductor substrate110. The structure112has been shows inFIG.1Ato be straight, parallel, micro-channels, and inFIG.1Bto be straight, parallel, micro-channels, non-straight channels, and/or reservoirs. Other shapes may be used, for example, trenches having non-parallel sides, etc. The formation of these structures112in the substrate110may be accomplished with any known method, in addition to the LIL method discussed above with regard toFIG.2. Any combination of such manufacturing methods may be used.

After the surface of the semiconductor substrate110is shaped to obtain the desired structures112/112-2/113/124/126, an ion solution120is pumped with the micro-pump140, from a container122, to the structures112/112-2/113/124/126. If the substrate110is made from a material which may interact (electrically or optically) with the final MWs132, then it is possible, in an optional step, to first coat the interior surface of the structure112/112-2/113/124/126with an oxide film, e.g., SiO2114, to insulate the two materials from each other. Other coatings may be added as deemed necessary. The ion solution120could be any perovskite material130, but also could include other materials, for example, an ion-crystal semiconductor material. A combination of perovskite materials may be used. The ion-crystal semiconductor material130is provided in a solution so that the capillarity of the micro-pump140takes the solution from the container122to the structures112/112-2/113/124/126. Note that the material130is mixed up with a solvent (e.g., DMSO) to form the ion solution120. In one embodiment, lead-halide-based perovskite material130is used for the ion solution120. In one optional step, the evaporation rate of the ion-crystal semiconductor material130is determined and the micro-pump140is sized to pump enough solution120to replace the evaporated solvent. The size of the micro-pump140dictates how much of the ion solution120is transferred from the container122to the structure112/112-2/113/124/126as the capillaries formed in the micro-pump140determine the amount of solution transferred. In one application, the capillaries are supplied by a paper filter144, which is held in place by a strong substrate142, for example glass. However, one skilled in the art would recognize that any material that is inexpensive and have natural capillaries would be able to act as the micro-pump. If the selected capillary material is strong enough, no substrate142is necessary.

In step1604, the ion solution120flows from a first end110A (proximal end) of the substrate110to a second end110B (distal end), which is opposite to the first end110A. The flow happens because of the narrow width of the structure112/112-2/113/124/126, which is in the range of 0.1-10 μm. In one application, the width of the structure is less than 1 μm, except for the reservoirs, which can be up to 100 μm. If large reservoirs are used, in one application, the substrate110may be tilted to promote the flow of the ion solution through the reservoirs. A height of the structure is between 1-10 μm. In one application, the height of the structure is less than 2 μm, for example, 1.5 μm. The distal ends of the structure112/112-2/113/124/126may be closed, so that when the ion solution120arrives at these ends, the flow of the ion solution at the distal end stops. Once this happens, the flow upstream the distal end happens only because of the evaporation of the solvent of the ion solution120. Thus, in this step, the flow of the ion solution is desired to just replace the evaporated solvent to avoid spills or interruptions in the fluid, which will be the origin for discontinuities and imperfections in the MWs to be formed.

In step1606, the ion-crystal semiconductor material130in the ion solution120starts to crystalize and form MWs132. If the material130includes perovskite, the MWs that are formed inside the structure are perovskite MWs. The MW follow the shape of the structure112/112-2/113. Thus, by controlling the shape and size of the structure112/112-2/113, the shape and the size of the MWs is controlled. Therefore, the MWs132can have a width and/or height in the μm range. However, in one embodiment, it is possible that the MWs132have a width and/or height in the nm range, if the structure112is so sized.

An interface810between the crystalized material, i.e., the MWs132and the ion solution120in a given micro-channel112moves from the distal end to the proximal end of the micro-channel112. In step1608, an evaporation rate of the ion solution120at the interface810may be controlled with an accelerate evaporation device800, which with an air jet moves faster the air above the interface810for facilitating the evaporation of the solvent. Thus, in step1608, it is possible to control the evaporation rate of the solvent in the ion solution120. Note that because the ion-crystal semiconductor material130is formed in the structure112/112-2/113/124/126, and there is no direct handling of the formed MWs132, there is no danger of breaking the MWs, especially when the MWs are brittle. Thus, in this embodiment, there is no movement or touching of the MWs while they are being made or after being made. In one application, the controlled evaporation rate is correlated with the amount of ion solution120that is pumped into the structure112. In other words, the size if the filter paper144is selected so that the ion solution carried by capillarity by the filter paper144substantially balances the evaporated solvent.

Once the MWs have been made, i.e., the perovskite material has crystalized and the MWs are solid, a first electrode1210is added in step1610to the proximal end of the structure112/112-2/113/124/126and a second electrode1212is added to the distal end of the structure112/112-2/113/124/126to form an electrical circuit. The ends of the electrodes may be used to connect to other elements formed on the substrate110, for example, a solar cell, a transistor, a PD, etc. depending on the needs. In one application, the exposed surface of the MWs may be covered with a protective and/or transparent material.

The disclosed embodiments provide a method for forming ion-crystal semiconductor material MWs embedded into a semiconductor platform based on a micro-pump fluidic strategy, and/or making a semiconductor device based on the perovskite MWs. It should be understood that this description is not intended to limit the invention. On the contrary, the embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

REFERENCES