• 专利附录
  • 专利说明
  • 权利要求
  • 类似技术
  • 同族专利
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  • 法律状态
  • 优先权
    • 专利摘要:
    Method of obtaining substrate-free metal flakes by means of a light-activated electroless deposition process. The invention consists of a method of manufacturing substrate-free metal flakes from a first electrochemical process and a subsequent chemical process. During the electrochemical process, illumination of the semiconductor substrate immersed in a solution of metal cations results in the reduction of these ions into metal atoms by electroless deposition on the semiconductor surface discontinuously, giving rise to flakes of thickness in the range of nanometers and an area of up to several square millimeters. The subsequent chemical process is used to attack the semiconductor, thus allowing the metal flakes to be released from the substrate. The method makes it possible to obtain flakes free of substrate in a fast, reproducible way, with a low cost and with equipment that can be easily transferred to the industry. (Machine-translation by Google Translate, not legally binding)
    公开号:ES2717347A1
    申请号:ES201900015
    申请日:2019-02-11
    公开日:2019-06-20
    发明作者:Sanchez Rocio Ranchal；Diaz Alicia Prados
    申请人:Universidad Complutense de Madrid；
    IPC主号:
    专利说明:

    [0001]
    [0002] Method of obtaining metal flakes free of substrate by electroless deposition process activated by light.
    [0003]
    [0004] Sector of the technique
    [0005]
    [0006] The present invention falls within the field of manufacturing nanomaterials; More specifically, the invention relates to a photoelectrochemical process based on the growth by electroless deposition, on a semiconductor substrate, of a metal in the form of flakes with a controlled thickness in the range of nanometers, followed by a release of the substrate to give rise to free flakes (known in English as free-standing flakes). These metallic flakes have application in the industry of electronic components.
    [0007]
    [0008] BACKGROUND OF THE INVENTION
    [0009]
    [0010] Flakes (or flakes, in English) are those two-dimensional material systems with a thickness from an atomic layer to hundreds of nanometers and with areas that can go up to hundreds of square microns or even square millimeters. His recent interest resides in that the properties of the materials change radically when reducing his thickness since that reduction of size allows to reach new properties like those derived from the topology of the physical systems. Therefore, there is a clear interest in the production of flakes.
    [0011]
    [0012] The methods of synthesis of metal flakes are, however, complex. Several methods have been described in the literature. Thus, metal flakes with nanometer thicknesses have been obtained by spraying metallic sheets immersed in a liquid medium by agitation maintained for several minutes or hours (US 2839378); silver and copper flakes have been obtained from ground silver and copper powder in a liquid medium such as propylene glycol and / or fatty acids with grinding times between 4 and 66 hours to obtain high conductivity pastes (US4331714, US7459007 and US4884754); a method for the production of metal flakes by means of a jet-type ejection process where part of the molten metal (WO2007020364) has also been described; and a method based on the metal scraping of pure copper sheets with a cutter to obtain copper flakes (WO2012171132).
    [0013]
    [0014] All the exposed methods have drawbacks such as the need for high temperatures or long synthesis times or difficulty of scaling. There is, therefore, a need to develop metal flake fabrication methods that are fast, low cost and scalable to the industry.
    [0015]
    [0016] In order to solve the deficiencies found so far, the present invention describes a photoelectrochemical method for obtaining flakes of metals (with a controlled thickness in the range of nanometers and with an area up to the square millimeters). semiconductor substrates that are subsequently released from the substrate to obtain free flakes. That is to say, a method is proposed to produce free-standing flakes of different metals from an electroless deposition process activated with light.
    [0017]
    [0018] Methods of growth of metals on different semiconductors by electroless deposition but in the form of powder or sheets have been described in the literature. Methods of manufacturing metals in the form of flakes on semiconducting substrates by electroless deposition or any other electrochemical technique have not been found.
    [0019] Explanation of the invention
    [0020]
    [0021] The method of obtaining substrate-free metallic flakes of the invention consists of performing an electroless deposition process on n-type semiconductor substrates with an electron density greater than or equal to 41016 electrons / cm 3. The semiconductor, in addition, must be stable in aqueous medium in dark conditions but undergo photo-corrosion when illuminated with an energy greater than the width of its banned energy band. Examples of these semiconductors are Si, GaAs, GaP, ZnO, CdS, InP, Cu 2 O or SiC.
    [0022]
    [0023] Prior to the electroless deposition process , one of the faces of the semiconductor substrate is prepared to achieve ohmic contact; the opposite side (on which the metal flakes will be deposited) is polished and then subjected to a treatment to obtain a clean surface without rust, preventing it from undergoing alterations by photo-corrosion (for this, the process is carried out in the dark or with less energy than the banned semiconductor energy band).
    [0024]
    [0025] Under these lighting conditions, the substrate is transferred to an electrochemical cell with an aqueous electrolyte containing cations of the metal to be electrodeposited. This electrolyte is called "growth electrolyte". The electrochemical cell consists of three electrodes: cathode (in this case it is the semiconductor substrate), anode and reference electrode. The electrodes are connected to a potentiostat with which the Open Circuit Potential (OCP) is recorded , which is the potential drop between the cathode and the reference electrode.
    [0026]
    [0027] Once submerged in the growth electrolyte, the substrate must remain for the time necessary for the substrate-electrolyte interface to stabilize and a stable OPC to be reached. It is essential that the OCP be more positive than the potential for reduction of the metal cations on the semiconductor in question. Said reduction potential depends on the temperature, the composition of the electrolyte and the semiconductor used, as well as its doping and the crystallographic orientation of its surface.
    [0028]
    [0029] The thickness of the metal flakes is controlled by the lighting time in such a way that the longer the light, the greater the thickness.
    [0030]
    [0031] Finally, the semiconducting substrate covered with metal flakes is immersed in darkness in a solution that attacks the substrate but not the metal deposited to obtain metal flakes free of substrate.
    [0032]
    [0033] BRIEF DESCRIPTION OF THE DRAWINGS
    [0034]
    [0035] Figure 1. Optical microscopy image obtained at 10 magnifications of a n-GaAs (111) B surface covered with Bi flakes after being illuminated for 8 minutes in the growth electrolyte.
    [0036]
    [0037] Figure 2.- Shows an atomic force microscopy image of a 200 nm thick Bi flake obtained after illuminating the n-GaAs (111) B substrate for 8 minutes in the growth electrolyte.
    [0038]
    [0039] Figure 3.- Bismuth diffraction pattern deposited by an electroless deposition process activated with light on a substrate n-GaAs (111) B.
    [0040]
    [0041] PREFERRED EMBODIMENT OF THE INVENTION
    [0042]
    [0043] The present invention is illustrated by the following examples, which are not limiting of its scope.
    [0044] Example 1.
    [0045]
    [0046] This example refers to the obtaining of Bi flakes on n-GaAs (111) B.
    [0047]
    [0048] The substrate n-GaAs (111) B is prepared previously following the routine described in the literature (A. Prados et al., J. Phys. Chem. C 122 (2018) 8874): initially, an ohmic contact is made in the of the substrate by thermal evaporation of 80 nm of an AuGe alloy (with 2% of Ge) followed by 250 nm of Au, which is subsequently heated to 380 ° C in Nidrón (95% N 2 and 5% H 2 ) for 90 s; subsequently, the substrate is degreased by washing in acetone and 2-isopropanol and then drying in an inert gas such as N 2 . Finally, to remove the native oxide from the polished surface of the substrate on which the Bi is to be deposited, the substrate is treated in the dark with several solutions: firstly, it is immersed in a solution at 10% by volume of HCl during 2 minutes; secondly, for 2 minutes in deionized water and, thirdly, 2 minutes in a 1 M solution of HCIO 4 .
    [0049]
    [0050] A growth electrolyte based on deionized water is prepared with 1 mM of Bi 2 O 3 and 1 M of HCIO 4 , in such a way that a strongly acid electrolyte is obtained (with a pH around 0.1).
    [0051]
    [0052] Then, the n-GaAs substrate free of oxide and with the undamaged surface is transferred to the electrochemical cell containing the growth electrolyte. During the transfer, the surface of the substrate is covered with a few drops of HCIO 4 to prevent it from oxidizing when exposed to air. A platinum mesh is used as the anode and an Ag / AgCl electrode (3M) as the reference electrode. The substrate is kept submerged in darkness for 1 to 2 minutes so that the substrate-electrolyte interface stabilizes and a stable OCP is reached. Then, the electrochemical cell is illuminated with light from an LED bulb with a color temperature Tc of 2700-3500 K, a power of 3 W and a luminosity of 210 Im. The bulb is placed about 20 cm from the n-GaAs substrate, illuminating it completely. The absorption of photons by the substrate simultaneously allows both the reduction of ions Bi (III) in metallic Bi and the photo-corrosion of the surface of n-GaAs, resulting in the formation of Bi flakes (Figure 1) with contours well defined (Figure 2). Under these conditions, a growth rate of 0.4 nm / s is achieved. With substrates n-GaAs (111) B the Bi flakes grow with a preferred crystalline orientation, with the planes (012) of the rhombohedral structure of the metallic Bi (R3m, 166) parallel to the planes (111) of the GaAs (Figure 3 ).
    [0053]
    [0054] Then, the substrate n-GaAs (111) B covered with Bi flakes was immersed in darkness for a few seconds in a solution of 0.5 MH 2 O 2 with pH = 11 and assisted by magnetic stirring to facilitate the attack of the n- GaAs and release the Bismuth flakes that are stable in this solution. The solution is subjected to a centrifugation to group the Bi flakes already free of substrate. Finally, the flakes can be collected with a pipette for later transfer.
    [0055]
    [0056] Example 2
    [0057]
    [0058] This example relates to the obtaining of Bi flakes on n-GaAs (110).
    [0059]
    [0060] The same steps are carried out as in Example 1 but using n-GaAs (110) as the substrate. Bismuth flakes with a different preferred crystalline orientation are obtained, with the planes (018) of the rhombohedral structure of the metallic Bi (R3m, 166) parallel to the planes (110) of the n-GaAs.
    [0061] Example 3
    [0062]
    [0063] This example refers to the obtaining of Bi flakes on n-GdS
    [0064]
    [0065] The same procedure described in Example 1 is carried out but using n-GdS substrate prepared as described in the literature (M.A. Elmorsi et al., Electrochim, Acta 31 (1986) 211). The ohmic contact is made by an alloy of In-Ga. The opposite surface is polished and then submerged in HCI for 10 seconds to remove the native oxide.
    [0066]
    [0067] The growth electrolyte is the same as that used in example 1. Once submerged in the growth electrolyte, the CdS is illuminated with a light of energy higher than 2.4 eV. Once the Bi flakes are deposited, the CdS substrate is immersed in a solution based on HCl and H 2 O 2 in the dark and subjected to magnetic agitation.
    [0068]
    [0069] Example 4
    [0070]
    [0071] This example refers to obtaining Ni flakes over n-Si.
    [0072]
    [0073] The same procedure described in Example 1 is again carried out using substrates of n-Si prepared as described in the literature. The ohmic contact is carried out by evaporation of a first layer of magnesium oxide and a second layer of aluminum (Y. Wan et al., Adv. Energ.Mater.7 (2016) 1601863). To remove the native oxide from the opposite surface, submerge the substrate in a solution consisting of 4 parts of concentrated H 2 SO 4 and 1 part of 30% H 2 O 2 at 80 ° C for 10 minutes. The substrate is rinsed with deionized water and immersed for 30 s in a 1% HF aqueous solution (D. Niwa et al., Electrochim, Acta 48 (2003) 1295). The growth electrolyte is composed of 2.3 M MSO 4 6 H 2 O 0.6 NO 2 6 H 2 O 0.5 MH 3 BO 3 with pH = 2 (F. Nasirpouri et al., Electroanal, Chem. 690 ( 2013) 136). Once immersed in the growth electrolyte, the Si illuminates with a light of energy higher than 1.12 eV. Once the Ni flakes are deposited, the Si substrate is immersed in a solution based on HF and H 2 O 2 in the dark and subjected to magnetic stirring.
    [0074]
    [0075] Example 5
    [0076]
    [0077] This example refers to obtaining Cu flakes on n-Si.
    [0078]
    [0079] The same steps are carried out as in the procedure described in example 4 but using a growth electrolyte composed of 7.5 mM CuCO 3 -Cu (OH) 2 with 20 g I-1 of H 3 BO 3 and 9.6 g I-1 HBF 4 at pH = 14 (G. Oskam et al., J. Phys. D: Appl. Phys. 31 (1998) 1927).
    权利要求:
    Claims (3)
    [1]
    1. Method of obtaining metal flakes comprising:
    - Select a n-type semiconductor substrate with an electron density greater than or equal to 410-6 electrons / cm3 stable in aqueous medium under dark conditions but experiencing corrosion under lighting conditions.
    - Prepare one of the faces of the selected substrate to achieve ohmic contact;
    polish the other side and then treat the face to obtain a clean surface without rust.
    - Transfer the substrate thus treated to an electrochemical cell with an aqueous electrolyte containing ions of the metal to be deposited on the substrate (growth electrolyte). The electrochemical cell consists of three electrodes: cathode (semiconductor substrate), anode and reference electrode, connected to a potentiostat with which the open cell potential (OCP) is recorded.
    - Keep the substrate submerged in the electrochemical cell until the substrate-electrolyte interface stabilizes and a stable OPC is reached and greater than the reduction potential of the metal cations on the semiconductor substrate.
    - Illuminate the electrochemical cell with energy light greater than the width of its banned energy band to modify the OPC so that it is more negative than the reduction potential of the metal cations on the semiconductor substrate.
    - Control the lighting time to achieve the desired snowflake thickness of the range of nanometers and with areas up to the square millimeters, so that the longer the illumination, the greater the thickness.
    [2]
    2. Method of obtaining metal flakes, according to claim 1, which further comprises releasing the flakes of the substrate by immersing in dark conditions the semiconductive substrate coated with flakes in a solution that attacks the substrate but does not attack the deposited metal.
    [3]
    3. Method for obtaining metal flakes free of substrate, according to previous claims, where the semiconductor substrate is Si, GaAs, GaP, ZnO, CdS, InP, Cu 2 O or SiC.
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