KR102831455B1 - PREPARING METHOD OF Ag-DOPED MERCURY TELLURIDE NANOCRYSTAL SOLUTION AND INFRARED OPTICAL APPARATUS USING THE SAME - Google Patents

Abstract

The present invention relates to a method for preparing a silver-doped mercury telluride nanocrystal solution, and an infrared optical device using the same, comprising the steps of (a) preparing a solution by adding an amine compound to a mercury precursor; and (b) injecting a tellurium precursor into the prepared solution, wherein a step of performing ligand exchange by adding a silver precursor in step (a) or adding a compound for ligand exchange and the silver precursor after step (b) is added.

Description

PREPARING METHOD OF Ag-DOPED MERCURY TELLURIDE NANOCRYSTAL SOLUTION AND INFRARED OPTICAL APPARATUS USING THE SAME

The present invention relates to a method for producing a silver-doped mercury telluride nanocrystal solution and an infrared optical device using the same, for responding to an infrared light source in the mid-infrared region.

With the growing demand for mid-infrared (MWIR) detectors in defense, agriculture, communications, advanced transportation, biomedical imaging, and environmental applications, cost reduction in MWIR optoelectronics has become crucial, as infrared materials are several times more expensive than visible-light counterparts. Following epitaxial growth materials and superlattice structures, colloidal quantum dots (CQDs) have emerged as a next-generation infrared-sensitive material due to their superior properties, including wavelength tunability, ease of synthesis, solution processability, and low-cost fabrication processes that do not require high-temperature procedures.

Over the past decade, CQD-based detectors have been progressively studied, focusing on Hg-based CQDs, which have a narrow bulk bandgap energy suitable for MWIR. Following the first report of a photoconductive MWIR detector based on HgTe CQDs, the Guyot-Sionnest team demonstrated a photovoltaic HgTe CQD-based mid-infrared (MIR) detector. A pn junction was constructed between HgTe (n-type) and Ag ₂ Te (p-type) CQD layers, where a solid-state cation exchange process with Hg cations in the QD film controlled the p-doping level of the Ag ₂ Te CQD layer. The photodiode structure suppressed noise and achieved commercially comparable performance, approaching background-limited infrared photodetection (BLIP). The success of the photovoltaic structure encouraged the CQD-based detector community, and several studies have followed to overcome limitations and achieve appropriate band alignment and enhanced absorbance by introducing electron/hole transport layers and novel photonic structures.

However, the following studies were limited to the initial HgTe CQD/solid cation-exchanged Ag ₂ Te CQD structure and focused on supplementary materials for further improvement rather than the chemical properties of the quantum dots that directly affect the formation of a proper p-n junction. Recently, Tang's group demonstrated a p-n homojunction by utilizing a solution-state ligand exchange process of HgTe CQDs to control the n-/p-type characteristics, which showed similar performance with enhanced infrared absorption due to the higher packing density of the homojunction even at 300 K. Other studies have shown that solution-state ligand exchange increases the carrier mobility of CQDs by up to 1-4 times. This suggests that chemically modifying the CQD properties plays a significant role in determining the performance, and suggests that further studies on the chemical control of CQDs are needed.

Although HgTe CQDs have been continuously studied in various ways, such as in terms of doping density and carrier mobility, p-type CQDs have been lacking, despite the importance of the doping state in forming the built-in potential. While widely used Hg-doped Ag ₂ Te CQDs have shown promising performance as a p-type doping material, further optimization and application have been hindered due to the extremely sophisticated process required for solid-state post-chemical processing to control the doping density of the p-doped material to the optimal concentration. Furthermore, due to the difference in doping density within the layer, a solid-state cation exchange process is always required after fabrication of n-doped QD films to form an appropriate p-n junction.

The present invention provides a method for producing a silver-doped tellurized mercury nanocrystal solution, which is sensitive to an infrared light source in the mid-infrared region, and comprises the steps of (a) preparing a solution by adding an amine compound to a mercury precursor; and (b) injecting a tellurium precursor into the prepared solution, wherein a step of performing ligand exchange by adding a silver precursor in step (a) or adding a compound for ligand exchange and the silver precursor after step (b) is added.

However, the technical problems to be solved by the present invention are not limited to the problems mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below.

The present invention provides a method for producing a silver-doped mercury telluride nanocrystal solution, which is sensitive to an infrared light source in the mid-infrared region, comprising the steps of (a) preparing a solution by adding an amine compound to a mercury precursor; and (b) injecting a tellurium precursor into the prepared solution, wherein a step of performing ligand exchange by adding a silver precursor in step (a) or adding a compound for ligand exchange and the silver precursor after step (b) is added.

The molar ratio of the silver precursor and the mercury precursor may be 1:1 to 1:5.

The above amine compound may include at least one selected from the group consisting of oleylamine, n-octylamine, aniline, 4,4'-bipyridine, p-phenylenediamine, ethylenediamine, and tris(2-aminoethyl)amine.

The tellurium precursor can be injected while the temperature of the solution prepared in the above step (b) is raised to 80°C to 120°C.

After injection of the tellurium precursor in the above step (b), a ligand mixture of a phosphine-based compound and a first thiol-based compound can be additionally injected.

Additional injection of the above ligand mixture can be performed in an ice bath at -10°C to 0°C.

In one embodiment of the present invention, an infrared optical device is provided, characterized in that a substrate; a telluride mercury nanocrystal layer; a silver-doped telluride mercury nanocrystal layer manufactured by the above method; and a metal electrode are sequentially laminated.

A second thiol-based ligand is bonded to the above mercury telluride, so that the mercury telluride nanocrystal layer can have n-type characteristics.

The silver-doped mercury telluride nanocrystal layer can have p-type characteristics by bonding a third thiol-based or halogenated ligand to the silver-doped mercury telluride.

The above infrared optical device can be sensitive to an infrared light source in the mid-infrared region.

The method for producing a silver-doped mercury telluride nanocrystal solution according to the present invention is characterized by comprising the steps of: preparing a solution by adding an amine-based compound to a mercury precursor and a silver precursor, and then injecting a tellurium precursor therein. Alternatively, the method for producing a silver-doped mercury telluride nanocrystal solution according to the present invention is characterized by the steps of: preparing a solution by adding an amine-based compound to a mercury precursor, then injecting a tellurium precursor therein, and then adding a silver precursor and a compound for ligand exchange to perform ligand exchange. That is, the present invention has the advantage of being simple and easy to produce by applying a solution-phase high-temperature injection method or a solution-phase cation exchange method.

Accordingly, the infrared optical device according to the present invention is characterized by sequentially stacking a substrate; a mercury telluride nanocrystal layer having n-type characteristics; a silver-doped mercury telluride nanocrystal layer having p-type characteristics; and a metal electrode, so that the film forming each nanocrystal layer is stable in air without agglomeration and has excellent sensitivity to an infrared light source in the mid-infrared region. Therefore, it can be usefully utilized scientifically, militaryally, and industrially.

Figure 1(a) is the FT-IR absorption spectrum of Ag-HgTe and DDAB-treated HgTe NCs, Figure 1(b) is the XRD pattern of Ag-HgTe and HgTe NCs with reference to cubic HgTe and monoclinic Ag ₂ Te, Figure 1(c) is the XPS spectrum of Hg 4f of HgTe and Ag-HgTe NCs, and Figure 1(d) is the TEM image of Ag-HgTe NCs (scale bar = 10 nm).
Fig. 2(a) is a schematic diagram of the sapphire/ITO/HgTe/Ag-HgTe/Au detector structure, Fig. 2(b) is a cross-sectional image of the HgTe CQD detector using FIB, and Fig. 2(c) is a band energy diagram.
Figure 3 shows the characteristics of HgTe CQD detectors with different Ag contents in the Ag-HgTe NC layer. A series of JV characteristics under darkness (dashed line) and light (300°C blackbody radiation, solid line) at (a) 78 K and (b) 130 K. (c) Responsivity and (d) open-circuit voltage as a function of operating temperature under 300°C blackbody radiation.
Figure 4(a) shows the reactivity of various ligands used in the Ag-HgTe NC layer, and Figure 4(b) shows the current density of the detector with different ligands at 78 K.
Figure 5(a) shows the JV characteristics under weak blackbody radiation intensities of 20°C and 35°C at 78 K, and Figure 5(b) shows the time-dependent current density curve derived from the blackbody radiation difference. The data were collected at a constant voltage of −0.005 V near 0 V. Figure 5(c) shows the JV curves under blackbody radiation of 30–45°C (inset: current density at 0 V with various temperatures), and Figure 5(d) shows a schematic diagram of the imaging measurement setup, and Figure 5(e) shows the actual image (left) and the corresponding thermal image (right) captured with the HgTe CQD detector at 78 K.

Infrared colloidal quantum dots (CQDs) have attracted interest due to their low fabrication cost and easy wavelength tunability for various infrared optoelectronic applications. Recently, mid-infrared (MWIR) quantum dot sensors have been successfully realized by forming photodiodes using chemical post-processing methods. Controlling the doping density of quantum dot solids and engineering the device architecture require extremely sophisticated techniques, which hinder consistent doping density and limit further development in understanding the fundamental photophysics and fabrication processes. Therefore, we report air-stable and highly reproducible MWIR CQD photodiodes incorporating newly synthesized p-doped Ag-HgTe nanocrystals (NCs). The Ag-HgTe alloy NCs clearly define the p-doped QD layer, ensuring a uniform dopant distribution and facilitating the fabrication of engineered devices. By optimizing the doping density, the inventors achieved a mean noise equivalent temperature difference (NETD) of 6 mK at 78 K, the lowest value ever reported, using a self-powered MWIR photodiode sensor.

In this specification, the term "mid-infrared region" broadly refers to the range from 2.5 μm to 8.0 μm, and more specifically, the range from 2.5 μm to 5.0 μm. Since the main absorption spectra of numerous molecules are concentrated in this mid-infrared region, infrared devices sensitive to infrared light sources in the mid-infrared region can be usefully utilized for scientific, military, and industrial purposes.

Hereinafter, the present invention will be described in detail.

Method for preparing a silver-doped mercury telluride nanocrystal solution

The present invention provides a method for preparing a silver-doped mercury telluride nanocrystal solution, comprising the steps of (a) preparing a solution by adding an amine compound to a mercury precursor; and (b) injecting a tellurium precursor into the prepared solution, wherein a step of performing ligand exchange by adding a silver precursor in step (a) or adding a compound for ligand exchange and the silver precursor after step (b) is added. That is, the method for preparing a silver-doped mercury telluride nanocrystal solution according to the present invention is characterized in that it is performed in the solution phase, and applies a solution-phase high-temperature injection method or a solution-phase cation exchange method.

First, the method for preparing a silver-doped mercury telluride nanocrystal solution according to the present invention includes a step [step (a)] of preparing a solution by adding an amine compound to a mercury precursor.

Optionally, for the solution-phase high-temperature injection method only, a silver precursor may be added together with the mercury precursor.

The molar ratio of the silver precursor and the mercury precursor may be 1:1 to 1:5, preferably 1:1 to 3:10, but is not limited thereto. Accordingly, the average atomic percentage of doped silver in the silver-doped mercury telluride may range from about 1.0% to about 10.5%, and as the average atomic percentage of doped silver in the silver-doped mercury telluride increases, the p-type characteristics may be improved.

As the silver precursor and mercury precursor, known compounds may be used. For example, the silver precursor may be silver chloride, and the mercury precursor may be mercury chloride. In addition, the amine compound may include at least one selected from the group consisting of oleylamine, n-octylamine, aniline, 4,4'-bipyridine, p-phenylenediamine, ethylenediamine, and tris(2-aminoethyl)amine, and is preferably oleylamine, but is not limited thereto.

Next, the method for preparing a silver-doped telluride mercury nanocrystal solution according to the present invention includes a step [step (b)] of injecting a tellurium precursor into the prepared solution.

A tellurium precursor can be injected while the temperature of the solution prepared above is raised to 80°C to 120°C. The tellurium precursor may be hydrophobic ligand-tellurium, and for example, the hydrophobic ligand may be a phosphine-based compound, such as trioctylphosphine.

After the injection of the tellurium precursor, a ligand mixture of a phosphine-based compound and a first thiol-based compound may be additionally injected, and the additional injection of the ligand mixture may be performed in an ice bath at -10°C to 0°C. For example, the ligand mixture of the phosphine-based compound and the first thiol-based compound may be a ligand mixture of trioctylphosphine and a thiol-based compound having 12 to 20 carbon atoms (particularly, 1-dodecanethiol). As such, a thiol-based compound having 12 to 20 carbon atoms has the advantage of being easy to undergo subsequent ligand substitution.

Optionally, only in the solution phase cation exchange method, a step of performing ligand exchange by adding a silver precursor and a compound for ligand exchange after injection of the tellurium precursor and additional injection of the ligand mixture may be additionally included.

Since the content and type of the above-mentioned silver precursor have been described above, a redundant description will be omitted. In addition, the compound for the ligand exchange corresponds to the amine-based compound and a material that is easy to ligand exchange with (e.g., oleylamine), thereby facilitating silver doping into mercury telluride.

It is preferable that the temperature during the above addition be maintained at 18°C to 100°C, and the ligand exchange is preferably performed for 1 minute to 120 minutes, but is not limited thereto.

As reviewed above, the method for producing a silver-doped mercury telluride nanocrystal solution according to the present invention is characterized by comprising the steps of: preparing a solution by adding an amine-based compound to a mercury precursor and a silver precursor, and then injecting a tellurium precursor thereto. Alternatively, the method for producing a silver-doped mercury telluride nanocrystal solution according to the present invention is characterized by comprising the steps of: preparing a solution by adding an amine-based compound to a mercury precursor, then injecting a tellurium precursor thereto, and then adding a silver precursor and a compound for ligand exchange to perform ligand exchange. That is, the present invention has the advantage of being simple and easy to produce by applying a solution-phase high-temperature injection method or a solution-phase cation exchange method.

Infrared optical devices

The present invention provides an infrared optical device characterized in that a substrate; a telluride mercury nanocrystal layer; a silver-doped telluride mercury nanocrystal layer manufactured by the above method; and a metal electrode are sequentially laminated.

First, the infrared optical device according to the present invention includes a substrate, which may be an ITO layer formed on glass or sapphire. At this time, the thickness of the substrate (particularly, the ITO layer) may be 30 nm to 150 nm.

Next, the infrared optical device according to the present invention includes a mercury telluride nanocrystal layer formed on a substrate. The mercury telluride nanocrystal layer can be manufactured by applying a solution-phase high-temperature injection method, and specifically, can be manufactured by adding an amine-based compound to a mercury precursor to prepare a solution, and then injecting a tellurium precursor into the solution.

Thereafter, in order to improve the n-type characteristics of the mercury telluride nanocrystal layer, a second thiol-based compound may be additionally treated to the mercury telluride to bind the second thiol-based ligand to the mercury telluride through solution-phase ligand exchange. The second thiol-based compound is characterized by having a relatively short length. The second thiol-based compound is a thiol-based compound having 2 to 6 carbon atoms, and may include at least one selected from the group consisting of ethanethiol, 1,2-ethanedithiol, 1-hexanethiol, 1,6-hexanedithiol, and 3-mercaptopropionic acid, and in particular, 1,2-ethanedithiol is preferable, but is not limited thereto.

Next, the infrared optical device according to the present invention includes a silver-doped mercury telluride nanocrystal layer formed on the mercury telluride nanocrystal layer. Since the method for manufacturing the silver-doped mercury telluride nanocrystal layer has been described above, a redundant description will be omitted.

Thereafter, the silver-doped mercury telluride nanocrystal layer can be further treated with a third thiol-based or halogenated compound to improve the p-type characteristics, thereby bonding the third thiol-based or halogenated ligand to the silver-doped mercury telluride through solution-phase ligand exchange. The third thiol compound is a thiol compound having 2 to 6 carbon atoms, and may include at least one selected from the group consisting of ethanethiol, 1,2-ethanedithiol, 1-hexanethiol, 1,6-hexanedithiol, and 3-mercaptopropionic acid. In order to alleviate aggressive ligand exchange, 1,6-hexanedithiol is particularly preferred, but is not limited thereto. This has the advantage of improving the uniformity of the film. Meanwhile, the halogenated compound is preferably at least one selected from the group consisting of cetyltrimethylammonium halide, trimethylammonium halide, methylammonium halide, ammonium halide, tetrabutylammonium halide, and butylammonium halide, but is not limited thereto. This has the advantage of providing robust stability, improving conductivity, and increasing responsiveness to optimize performance.

Next, the infrared optical device according to the present invention includes a metal electrode formed on the silver-doped mercury telluride nanocrystal layer.

Additionally, the metal electrode may include at least one selected from the group consisting of silver (Ag), aluminum (Al), and gold (Au). At this time, the thickness of the metal electrode may be 30 nm to 200 nm.

Accordingly, the infrared optical device according to the present invention is characterized by sequentially stacking a substrate; a mercury telluride nanocrystal layer having n-type characteristics; a silver-doped mercury telluride nanocrystal layer having p-type characteristics; and a metal electrode, so that the film forming each nanocrystal layer is stable in air without agglomeration and has excellent sensitivity to an infrared light source in the mid-infrared region. Therefore, it can be usefully utilized scientifically, militaryally, and industrially.

Hereinafter, preferred examples are presented to aid in understanding the present invention. However, the following examples are provided solely to facilitate a better understanding of the present invention, and the scope of the present invention is not limited by the following examples.

[Example]

HgTe NC synthesis

A 200 mL three-necked flask was charged with 1.5 mmol of HgCl ₂ and 50 mL of OLAM, and the mixture was degassed under vacuum (~100 mTorr) at 100°C for 1 h. For the tellurium precursor stock solution (1 M TOP-Te), 10 mmol of tellurium powder was dissolved in 10 mL of TOP with vigorous stirring in a glovebox at 100°C for 1 h. After degassing the Hg precursor solution, the temperature of the flask was set to 97°C under an argon flow. 1.5 mL of the 1 M TOP-Te solution was rapidly injected into the three-necked flask, and after the desired reaction time, 4 mL of DDT and 4 mL of TOP were injected by ice-bath quenching. The product solution was centrifuged with chloroform to remove byproducts. The supernatant was precipitated with ethanol, and the precipitate was redispersed in chlorobenzene. The redispersed HgTe was added to a vial containing didodecyldimethylammonium bromide (DDAB) and stirred for 45 minutes to remove excess ligand.

Ag-HgTe NC synthesis

Ag-HgTe NCs were prepared using a similar method to HgTe NCs.

First, 0.5 or 1.5 mmol of AgCl powder, 1.5 mmol of HgCl ₂ , and 50 mL of OLAM were dissolved in a 200 mL three-necked flask using the high-temperature injection method. After degassing, 4 mL of a 1 M TOP-Te solution was rapidly injected into the flask at 97 °C under argon gas. The reaction proceeded for 1 minute and 30 seconds, after which a mixed solution of ligands (DDT and TOP) was injected by ice-bath quenching. The precipitation process was the same as that for HgTe NCs.

Alternatively, by cation exchange method, 1.5 mmol of HgCl ₂ and 50 mL of OLAM were dissolved in a 200 mL three-necked flask. After degassing, 4 mL of 1 M TOP-Te solution was rapidly injected into the flask at 97 °C under argon gas. The reaction was carried out for 1 minute and 30 seconds, and then a ligand mixture solution (DDT and TOP) was injected with ice-bath quenching. The precipitation process was the same as that of HgTe NCs. The precipitated HgTe NCs were then dispersed in chlorobenzene. 0.5 or 1.5 mmol of AgCl powder was dissolved in OLAM solution and toluene to prepare a silver precursor solution, which was then rapidly injected into the HgTe NC solution dispersed in chlorobenzene at 25 °C under argon gas to perform ligand exchange for 30 minutes. This was followed by quenching with chloroform and ethanol. The precipitation process was the same as that of HgTe NCs.

Device manufacturing

A 0.65 mm thick sapphire substrate (11 mm × 11 mm) was prepared. Indium tin oxide (ITO) was sputtered onto the substrate to a thickness of 70 nm as an electrode. The ITO-sapphire substrate was cleaned by sonication in distilled water, chloroform, acetone, and IPA. The dried substrate was treated with a 1% (v/v) MPTMS/toluene solution for 30 s and rinsed with IPA. 30 μL of HgTe NCs was dropped and spread over the entire surface of the substrate by handling. After annealing the NC film at 50°C, it was immersed in EDT/HCl/IPA (1:1:100 by volume). After ligand exchange, the film was rinsed three times with IPA. This process was repeated 3–7 times to fabricate an active layer film. For the p-type layer, 30 μL of Ag-HgTe NCs was used in the same manner as for the HgTe layer. EDT, HDT, and CTAB ligands were each dissolved in IPA to compare the ligand exchange effects. The p-type layer process was repeated 1–2 times. Finally, a Au electrode (80 nm) was deposited using a thermal evaporator.

Fourier transform infrared absorption

FTIR absorption spectra were measured using a Nicolet iS10 FT-IR with a resolution of 0.482 cm ^-1 .

X-ray diffraction

XRD patterns were measured using a Rigaku Ultima III X-ray diffractometer with graphite monochromated Cu Kα (λ=1.54056Å). The irradiation power was 40 kV at 30 mA. The sampling width was 0.01˚.

High-resolution analytical transmission electron microscopy

The morphology of HgTe and Ag-HgTe NCs was investigated using a Tecnai 20 model using an accelerating voltage of 200 kV.

X-ray photoelectron spectroscopy

XPS spectra were measured using a K-alpha model with a monochromatic Al X-ray source (Al Kα line: 1486.6 eV). The XPS spectra were calibrated with C 1s (284.8 eV) values. The atomic % of NC was determined from the XPS spectra.

UV photoelectron spectroscopy

UPS spectra were collected by a Nexsa ^TM X-ray photoelectron spectrometer with the He(I) line at an energy of 21.22 eV. The energy step size was set to 0.050 eV.

focused ion beam

Cross-sectional samples of the fabricated devices were imaged using an FEI Helious G5 UC focused ion beam.

field effect transistor

A heavily n-doped silicon wafer with 300 nm thick SiO ₂ was used as a substrate for a back-gate thin-film transistor (TFT). After cleaning with an organic solvent, Ti (5 nm)/Au (50 nm) interdigitated electrodes were thermally deposited, and the wafer was diced to 3 cm × 3 cm. The channel width and length of the interdigitated electrodes were 5.28 mm and 10 μm, respectively. Each NC was drop-casted and annealed at 50°C. The film was treated with CTAB or HCl/EDT solution for ligand exchange and washed with IPA. The process was repeated 1–2 times. The transfer characteristics of the FET were measured using a semiconductor parameter analyzer (4200A-SCS).

J-V measurement

For the electrical characterization of the device, the current density-voltage spectrum was measured using a semiconductor parameter analyzer (4200A-SCS). The fabricated device was placed in a Janis ST-100 optical cryostat with a CaF ₂ window to reduce the operating temperature. To verify the photosensitivity of the device, a calibrated blackbody (Omega BB703, 300°C) was used as an infrared illumination source. The J _SC value was defined as the current density value at zero voltage.

Noise spectral density measurement

To measure noise spectral density, the device was connected to a cryostat and placed in a custom-built Faraday cage to reduce exposure to external noise. The cryostat temperature was lowered to 78 K. A low-noise current preamplifier (Stanford Research Systems, SR570) was connected to the device to amplify the current signal into voltage units. For frequency-dependent voltage RMS spectra, an FFT dynamic signal analyzer (Stanford Research Systems, SR785) was used.

NETD measurement

NETD was calculated from the time-dependent current density spectrum from a semiconductor parameter analyzer (4200A-SCS). A constant voltage of -0.005 V was applied to the device. A total of 4,097 scan points were measured over a 388-second measurement period. Blackbody radiation (manufactured by I3 Systems) at 20°C was maintained for 150 seconds and then increased to 35°C in 80 seconds. The root mean square (RMS) value was calculated from the current density recording at 20°C.

Infrared image measurement

A single pixel of the fabricated device was used to scan the corresponding infrared image of the shadow mask. The mask was placed on an X-Y scanner, and blackbody radiation (Thorlab, SLS303) passed through the hole in the mask. Since the device was operated with low-temperature blackbody radiation, the shutter of the SLS303 was used in the closed state. A scanning lens was mounted in front of the cryostat to project the image through a detector. The device was operated at 78 K with a constant voltage of 0.01 V from a low-noise current preamplifier (Standford Research Systems, SR570). The photocurrent values were recorded by moving the mask with the X-Y scanner. The number of points to construct a 50 mm × 40 mm thermal image was 101 horizontally and 81 vertically.

Figure 1 provides the physical properties of Ag-HgTe and HgTe NCs. For p-type materials, previous reports on mid-infrared CQD photovoltaic detectors primarily utilized a solid-state cation exchange method by applying a mercury precursor solution onto Ag ₂ Te CQDs to form a p-type Hg-doped Ag ₂ Te CQD layer. Although subsequent cation exchange methods showed high detection rates comparable to commercial detectors, there was a risk of film cracking, which limited the application of p-type materials. To overcome these limitations, the present inventors synthesized Ag-HgTe alloy NCs.

Briefly, metal precursors of AgCl(s) and HgCl ₂ (s) were simultaneously added to an oleylamine (OLAM) solvent, and the mixture was degassed under vacuum to remove water and oxygen. After degassing, the solution temperature was set to 97°C, and then trioctylphosphine-tellurium (TOP-Te) solution was rapidly injected into the flask. The reaction was allowed to proceed for the desired reaction time, and the reaction was quenched in an ice bath by adding additional ligands, TOP and 1-dodecanethiol (DDT).

Figure 1(a) shows the absorption spectra of Ag-HgTe and HgTe NCs. The sharp peak at 2922 cm ^-1 corresponds to the vibrational modes of CH stretching in the DDT, TOP, and OLAM ligands that passivate the nanocrystal surface. The bandgap exciton peaks of Ag-HgTe and HgTe NCs were observed at ~3600 and ~3000 cm ^-1 . The less pronounced bandgap absorption feature of Ag-HgTe NCs was attributed to mixing with the CH vibrational modes (asymmetric and symmetric) of the surface ligands. The bandgap feature became clear after the ligands were exchanged for shorter ones. The bandgap absorption of HgTe NCs was redshifted to 2000 cm ^-1 by treatment with 1,2-ethanedithiol (EDT) ligands, which is suitable for mid-IR detection. This redshift occurred due to the overlap of the wavefunctions of the NCs as the interpoint distance decreased.

The XRD pattern indicated that the crystal structure of HgTe NCs was a zinc mixed cubic phase with distinct (111), (220), and (311) planes (Fig. 1(b)). The lattice spacing of 0.37 nm corresponded to the (111) plane, which corresponded to the zinc mixed structure of HgTe (Fig. 1(b)). The XRD pattern of Ag-HgTe NCs demonstrated the Ag doping of HgTe NCs, showing a major peak associated with cubic HgTe and a minor peak associated with monoclinic Ag ₂ Te. The zinc mixed peak of Ag-HgTe was slightly shifted to lower 2θ values, indicating that the doped NCs expanded to higher lattice parameters by incorporating Ag into the HgTe lattice (Fig. 1(b)). The lattice parameters of Ag-HgTe and HgTe NCs were calculated by Bragg's law, and the results showed that the lattice parameter of Ag-HgTe NCs was 6.452 nm, which was larger than that of HgTe NCs (6.432 nm). The XRD pattern of Ag-HgTe in Fig. 1(b) was associated with 10.5% Ag doping, which was described later, and was compared with the pattern of 2.0% Ag-doped HgTe.

Additionally, X-ray photoelectron spectroscopy (XPS) spectra of HgTe and Ag-HgTe NCs showed differences in binding energies depending on the Ag dopant (Fig. 1(c)). The two Hg 4f peaks representing Hg-Te and Hg-S bonding states were shifted to a lower binding energy of 0.47 eV in Ag-HgTe NCs compared to HgTe NCs. Similarly, the Te 3d peak of Ag-HgTe NCs was also located at a lower binding energy than that of HgTe NCs. This demonstrated that the chemical interaction between Hg cations and Te anions was altered due to Ag bonding.

TEM images showed an elongated morphology of Ag-HgTe NCs, which was different from the tetrahedral shape of HgTe NCs (Fig. 1(d)). The average length and width were ~11.2 and ~4.5 nm, respectively, indicating an aspect ratio of ~2.5. It is well known that the (111) plane plays a dominant role in forming the elongated morphology, and nanowires are generally grown along the (111) plane. While fast Fourier transform (FFT) analysis of HgTe showed uniform interplanar distances associated with the three major planes of cubic HgTe (3.7 Å, 2.3 Å, and 1.9 Å, corresponding to the (111), (220), and (311) planes, respectively), the interplanar distance of Ag-HgTe was 3.7 Å, which corresponds to the (111) plane, as evidenced by the TEM images, indicating that an interplanar distance of 3.7 Å was observed in Ag-HgTe NCs. Additionally, TEM images of Ag-HgTe NCs accompanied by energy dispersive spectroscopy (EDS) mapping showed a uniform distribution of Ag, Hg, and Te atoms.

Considering that the valence electrons of Hg and Ag atoms are 2 and 1, respectively, replacing Hg atoms with Ag atoms resulted in electron-deficiency bonding, leading to p-doping characteristics. To confirm the doping characteristics, field-effect transistor measurements were performed. The conductance of Ag-HgTe NCs decreased with increasing gate potential, demonstrating the p-type nature of Ag-HgTe NCs. In contrast, HgTe NCs exhibited the opposite behavior, which is characteristic of n-type materials. Therefore, HgTe and Ag-HgTe NCs are suitable as n-type and p-type materials for photodiodes, respectively.

The structure of the fabricated MWIR NC photodiode was sapphire/ITO/HgTe/Ag-HgTe/Au. Its schematic is shown in Fig. 2(a). The sapphire substrate and ITO electrode were used because of their relatively high transparency in the infrared region. HgTe and Ag-HgTe NC layers were deposited layer-by-layer (LBL) using the LBL method. The HgTe film was treated with EDT/HCl to enhance the n-type characteristic. The image of the film, in which a 440 nm thick HgTe NC layer was deposited using a cross-section focused ion beam (FIB), showed that the final thicknesses of the n-type and p-type layers were 440 nm and 45 nm, respectively, as shown in the FIB image of the device (Fig. 2(b)). The cross-section of the NC layer deposited using the LBL method was extremely smooth, which improved the performance of the device.

Since the work function of ITO is reported to be between 4.5 and 4.7 eV, it is compatible with the conduction band position of HgTe NCs (4.5 eV), minimizing the barrier to electron transport and operating as an appropriate electronic contact. Figure 2(c) shows the band alignment of the HgTe CQD detector analyzed by ultraviolet photoelectron spectroscopy (UPS). The conduction band offset between HgTe and Ag-HgTe NCs effectively prevented electrons from flowing in the opposite direction. For the top electrode, Au was used because of its function as an excellent back reflector for infrared radiation.

The performance of the fabricated photodiode device in Fig. 2 was tested at 78 K and 130 K (Fig. 3(a) and (b)). These devices exhibited clear rectification characteristics, demonstrating that a built-in potential was well formed at the junction between the n- and p-doped materials. Regarding the rectification stability, current density-voltage (J-V) curves were measured at various temperatures from 78 to 240 K. The rectification characteristics were clearly maintained up to 150 K, but the leakage current increased when the temperature exceeded 150 K.

Responsiveness was calculated using the following equation.

(1)

Here, J is the photocurrent density and P is the power density of the light source (21.7 mW/cm ² at a 300°C blackbody). The highest responsivity was 0.31 and 0.58 A/W at 78 K and 130 K, respectively. Considering the importance of high-temperature operating photodetectors, it is worth noting that the responsivity is higher at 130 K than at 78 K.

The Ag doping effect was further studied at various temperatures by controlling the Ag precursor to Hg ratio. The actual Ag content in each Ag-HgTe was analyzed by inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectrometry (ICP-OES). When the Ag precursor to Hg ratio was 1, the average atomic percentage of Ag was 10.5%, whereas when the Ag precursor to Hg ratio was 0.3, Ag was 2.0%. Hereinafter, Ag-HgTe containing 10.5% Ag is referred to as "10.5% Ag," and Ag-HgTe containing 2.0% Ag is referred to as "2.0% Ag." It should be noted that the synthesis and fabrication methods were identical, with the Hg to Ag precursor ratio being the only variable.

Figure 3(c) shows the temperature-dependent open-circuit voltage (V OC ) of devices fabricated with 2.0% and 10.5% Ag-HgTe NCs. V _OC showed a linear relationship with the temperature as given below.

(2)

Here, E _a is the activation energy, n is the diode ideality factor, k is the Boltzmann constant, T is the temperature, J _SC is the photocurrent, and J ₀₀ is the reverse saturation current prefactor. At 0 K, V _OC should be close to the band gap energy, but the V _OC of the 10.5% Ag and 2.0% Ag-HgTe devices showed values lower than the band gap energy of HgTe NCs (~0.36 eV). This meant that the open-circuit voltage deficiency was caused by energy disorder, which expanded the density of states and non-radiative recombination. According to Equation (1), a higher J _SC leads to a higher V _OC , which was demonstrated in the experimental results between the 10.5% Ag and 2.0% Ag-HgTe devices.

Temperature-dependent responsivity was measured for devices fabricated with 10.5% Ag and 2.0% Ag-HgTe NCs (Fig. 3(d)). 10.5% Ag-HgTe NCs exhibited higher performance than 2.0% Ag-HgTe NCs, and the responsivity difference was approximately twofold at 120–140 K. To investigate the difference between 2.0% and 10.5% Ag-HgTe NCs, ultraviolet photoelectron spectroscopy (UPS) was performed to determine the Fermi level position. The Fermi level and valence band edge were separated by 0.22 eV in 10.5% Ag-HgTe NCs, whereas they were separated by 0.32 eV in 2.0% Ag-HgTe NCs, indicating that the Fermi level of 10.5% Ag-HgTe NCs is much closer to the valence band than that of 2.0% Ag-HgTe NCs. UPS results confirmed that higher Ag content enhanced the p-type characteristics. Furthermore, when the Ag to Hg precursor ratio was varied in detail from 1:0.3 to 1:1, the response improved with increasing Ag precursor ratio. The strong p-doping of the Ag-HgTe NCs played a crucial role in forming an appropriate built-in potential, resulting in significant performance enhancement. The highest performance was achieved at 10.5% Ag, which was used as the p-type material for further optimization.

Along with the doping density, which determines the intrinsic band levels of the material, ligand exchange, an essential step in promoting charge transport, substantially modified the band alignment in the device. Because NCs are surface-sensitive, different interactions with the atoms constituting each ligand altered the surface dipole moment, which shifted the conduction or valence band positions relative to the environmental Fermi level. Furthermore, ligands could act as fillers for defect sites, passivating trap sites and ultimately altering the electron density. Changing the ligand type alone has been reported to result in a shift of nearly 1 eV in the band levels. Furthermore, ligands significantly impact device performance and stability by affecting film uniformity and the surface of the NC layer. While conventional EDTs have demonstrated promising performance, the high reactivity of short alkyl thiol ligands can induce rapid and aggressive exchange processes, resulting in partially aggregated films. Furthermore, organic molecules are susceptible to oxidation and heat, which directly impact the air stability and lifetime of the device. Therefore, exploring different ligands is necessary for achieving better and longer-lasting performance.

For comparison with the conventional ligand, EDT, 1,6-hexanedithiol (HDT) and cetyltrimethylammonium bromide (CTAB) were selected. HDT, due to its long organic chain, was expected to mitigate the aggressive exchange process and contribute to film uniformity. On the other hand, CTAB, as an inorganic ligand, could provide more robust environmental stability to the NC layer and enhance the wavefunction overlap of the NC, resulting in better conductivity. To verify the difference, all devices were fabricated using the same method using different ligands for the previously optimized p-type material (10.5% Ag-HgTe NC), and each response was the average of 21 devices.

Figures 4(a) and (b) show the average response and J-V curves of devices using three different ligands, along with their standard deviations. The results show that the use of CTAB can nearly double the response compared to EDT. The specific detection rate of the CTAB-treated device is given by the following equation.

(3)

Here, N is the spectral noise density (0.5 pA/Hz ^1/2 ), R is the responsivity (0.58 A/W), and A is the area of the detector (1.05 mm ² ). Based on spectral noise density measurements, the highest specific detectivity was 1.15Х10 ¹¹ Jones at 130 K.

To investigate the differences, we performed UPS on the ligand-treated NC films to closely examine the band energy levels. For accurate comparison, the films were fabricated with the same thickness as the devices. UPS spectra demonstrated that the energy levels of the EDT-treated Ag-HgTe NC layer were somewhat shallower, with the valence band being similar to the conduction band of the HgTe NC layer. In contrast, the valence band of the CTAB-treated Ag-HgTe NC layer (10.5% Ag) was 0.33 eV deeper than the conduction band of the HgTe NC layer, which was more favorable for forming a p-n junction.

In addition to the various band alignments, the better responsivity was attributed to the more stable surface passivation by the bromide atom ligands. Unlike divalent ligands such as EDT and HDT, which can create a negatively charged surface, the monovalent bromide ion could bind to each cation on the NC surface, providing a charge-neutral surface. Therefore, it contributed to effective defect passivation by interfering with unwanted recombination pathways. Furthermore, the bromide ligand exhibited a slower exchange rate than EDT, while its atomic configuration effectively shortened the interparticle spacing by 0.1 nm compared to EDT. This indicated that the NC could form a more conductive, less-agglomerated film, which lowers the energy barrier for electrons to move toward the electrode.

Despite the overall higher responsivity, devices with CTAB exhibited a large standard deviation in responsivity between different devices. This could be attributed to the solid-state ligand exchange process, which uses alcohol to wash away residual ligands. Because halides are susceptible to protonation, the alcohol wash process can desorb halide ions from the NC surface, forming trap sites. However, even with the lowest responsivity, the device using CTAB surpassed the average responsivity of devices using EDT and HDT, demonstrating the promising role of CTAB when the exchange process is further optimized. Furthermore, HDT exhibited relatively higher responsivity than EDT due to its mild ligand exchange rate. Nevertheless, the improvement was not significant because longer organic chains can form a higher energy barrier between NCs, reiterating the superior performance of short-atom CTAB ligands. Furthermore, CTAB-treated devices exhibited consistent rectification characteristics across all cells in a single device, demonstrating that CTAB treatment can provide uniform ligand exchange.

The Noise Equivalent Temperature Difference (NETD) of an infrared detector is an essential parameter that indicates its sensitivity to small temperature changes. A low NETD value allows the infrared camera to operate with high thermal resolution. The CTAB-treated Ag-HgTe device showed a noticeable difference in J _SC values under blackbody radiation at 20°C and 35°C (Figs. 5(a), (b)). The photocurrent according to the blackbody radiation temperature change is shown in Fig. 6(b), and the NETD was calculated as follows.

(4)

Here, ΔT is the temperature difference, ΔJ _s is the difference in photocurrent density measured between two blackbody radiances, and J _n is the RMS noise calculated from the J _SC values at a given temperature. ΔT is 15°C. ΔJ _s is 0.13 mA/cm ² observed from the time-dependent current density curve. J _n of 2.381Х10 ^-8 A/cm ² is calculated from the J SC values at 20°C. Here, the NETD value was 3 mK, which is the best result compared to previous reports of HgTe-based photodetectors. Considering the theoretical NETD equation,

(5)

Here, A is the device area and D ^* is the detection rate. The significantly lower NETD may be partly due to the large area (1.05 ^mm2 ) of the device fabricated here. NETD Since the area of the device studied here is relatively large compared to other studies, it is reasonable to indicate the NETD as low as 3 mK. The low NETD value is further evidenced by the JV curve, which is distinguished by its responsiveness to blackbody radiation at 1°C (Fig. 5(c)).

Along with low NETD value, high stability under voltage and varying temperature is essential for practical use. The stability of device performance was tested through 100 measurements at bias voltages ranging from -0.3 V to 0.3 V and temperatures varying from 78 K to 300 K. The measurement procedure was as follows. 1) At 78 K, the device was swept three times with different blackbody radiations (20 °C, 35 °C, 300 °C), and the same procedure was repeated at 150 K. A total of six scans were measured. 2) The temperature was raised to RT (300 K) in vacuum and then lowered back to 78 K. 3) Steps 1 and 2 were repeated 18 times, resulting in a total of 108 sweeps. When the response value of the first measurement was set to 100%, the device performance was maintained up to about 80% even after 103 sweeps, indicating excellent stability over various voltages and temperatures.

Finally, the performance of the CTAB-treated Ag-HgTe detector was visually verified by scanning the thermal image. The scanning imaging system for infrared imaging of the CTAB-treated Ag-HgTe detector is illustrated in Fig. 5(d), and the details of the imaging setup are summarized in the experimental section. The thermal image of the mask was acquired with a single pixel of the detector, and the image showed clear features of the mask (Fig. 5(e)). Blackbody radiation passing through the hole in the mask was projected onto the detector, and the photocurrent generated at the pixel was recorded to construct the infrared image. The number of points for constructing a 50 mm × 40 mm thermal image was 101 horizontally and 81 vertically. Since the p-CTAB-treated Ag-HgTe/n-HgTe QD detector can distinguish a difference in thermal radiation of 3 mK, the HgTe CQD-based detector showed excellent potential to replace MCT or InSb-based epitaxial semiconductor detectors.

In summary, we successfully fabricated a MWIR CQD-based photodetector with a record-breaking NETD of 3 mK at 78 K. We synthesized p-doped Ag-HgTe NCs using a one-pot synthesis method. This facile synthesis method allowed for controllable silver doping, which resulted in appropriate band alignment in the device. Furthermore, as the silver doping ratio increased, the p-type characteristics of the Ag-HgTe NCs improved, and the responsivity reached 0.58 A/W at 130 K. Ligand exchange, which determines the conductivity of the QD film, was key to improving device performance. Monovalent bromide ions passivated the surface of the Ag-HgTe NCs, preventing film aggregation. As a result, the p-CTAB-treated Ag-HgTe/n-HgTe QD device achieved the highest NETD value compared to conventional HgTe CQD-based photodetectors. This was attributed to the effective noise suppression achieved by fabricating a well-distributed CQD layer with an extremely smooth QD layer surface and a large device area. Thermal images were successfully acquired from a single pixel of the device. The results of the HgTe CQD-based device using Ag-HgTe p-type material showed great potential for application in low-cost and high-sensitivity MWIR thermal imaging cameras.

The foregoing description of the present invention is for illustrative purposes only. Those skilled in the art will readily appreciate that modifications to other specific embodiments can be made without altering the technical spirit or essential characteristics of the present invention. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive.

Claims (10)

(a) a step of preparing a solution by adding an amine compound to a mercury precursor; and
(b) comprising a step of injecting a tellurium precursor into the solution prepared above,
A method for producing a silver-doped mercury telluride nanocrystal solution, characterized in that a step of performing ligand exchange is added by adding a silver precursor in the step (a) above or by adding a compound for ligand exchange and a silver precursor after the step (b).
In the first paragraph,
A method for producing a silver-doped mercury telluride nanocrystal solution, characterized in that the molar ratio of the silver precursor and the mercury precursor is 1:1 to 1:5.
In the first paragraph,
A method for producing a silver-doped mercury telluride nanocrystal solution, characterized in that the amine compound comprises at least one selected from the group consisting of oleylamine, n-octylamine, aniline, 4,4'-bipyridine, p-phenylenediamine, ethylenediamine, and tris(2-aminoethyl)amine.
In the first paragraph,
A method for producing a silver-doped mercury telluride nanocrystal solution, characterized in that a tellurium precursor is injected while the temperature of the solution produced in the above step (b) is raised to 80°C to 120°C.
In the first paragraph,
A method for producing a silver-doped mercury telluride nanocrystal solution, characterized in that after injection of a tellurium precursor in the above step (b), a ligand mixture of a phosphine-based compound and a first thiol-based compound is additionally injected.
In paragraph 5,
A method for producing a silver-doped mercury telluride nanocrystal solution, characterized in that the additional injection of the above ligand mixture is performed in an ice bath at -10°C to 0°C.
Steps to prepare the substrate;
A step of depositing a telluride mercury nanocrystal layer on the above substrate;
A step of preparing a silver-doped mercury telluride nanocrystal solution using a method according to any one of claims 1 to 6, and then treating the solution on the telluride mercury nanocrystal layer to deposit a silver-doped mercury telluride nanocrystal layer; and
A method for manufacturing an infrared optical device, characterized in that it comprises a step of depositing a metal electrode on the silver-doped mercury telluride nanocrystal layer.
In paragraph 7,
A method for manufacturing an infrared optical device, characterized in that a second thiol-based ligand is bonded to the above mercury telluride, and the above mercury telluride nanocrystal layer has n-type characteristics.
In paragraph 7,
A method for manufacturing an infrared optical device, characterized in that a third thiol-based or halogenated ligand is bonded to the silver-doped mercury telluride, so that the silver-doped mercury telluride nanocrystal layer has p-type characteristics.
In paragraph 7,
A method for manufacturing an infrared optical device, characterized in that the infrared optical device is sensitive to an infrared light source in the mid-infrared region.