Abstract:
A method for fabricating metallic micro-nanostructures, the method including: 1) heating a metal and a micro-nanostructure mold to a temperature T, where the metal is a pure metal or an alloy thereof selected from indium (In), germanium (Ge), tin (Sn), bismuth (Bi), lead (Pb), zinc (Zn), aluminum (Al), copper (Cu), gold (Au), silver (Ag), platinum (Pt) and palladium (Pd), T is greater than or equal to 0.5 T m  and less than T m , with T representing an absolute temperature and T m  representing a melting point temperature of the metal at absolute temperature scale; 2) applying load to press the metal at the temperature T into the mold to obtain a composite structure comprising the mold and the metal; and 3) removing the mold, to obtain the metal with replicated micro-nanostructures.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Pursuant to 35 U.S.C. §119 and the Paris Convention Treaty, this application claims the benefit of Chinese Patent Application No. 201610518239.7 filed Jul. 4, 2016, the contents of which are incorporated herein by reference. Inquiries from the public to applicants or assignees concerning this document or the related applications should be directed to: Matthias Scholl P. C., Attn.: Dr. Matthias Scholl Esq., 245 First Street, 18th Floor, and Cambridge, Mass. 02142. 
       BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0002]    The invention relates to a method for fabricating metallic micro-nanostructures. 
       Description of the Related Art 
       [0003]    Micro-nanostructured materials have significantly improved optical, electrical, magnetic, and catalytic properties, and thus have been widely applied in many fields, such as, e.g., batteries, catalysis, optics, sensing and surface physical chemistry. 
         [0004]    Methods for preparing metallic micro-nanostructures can be classified into two groups: one is a “top-down” method, i.e., obtaining micro-nanostructures from bulk materials, the other is a “bottom-up” method, including: liquid phase synthesis, thermal evaporation, sputtering, electrodeposition, and chemical deposition. 
         [0005]    The aforesaid methods are complex and inefficient, require expensive equipment, and are not able to achieve good control of the micro-nanostructures size. Although the thermoplastic nanoimprinting method provides an inexpensive and fast way to prepare ordered micro-nanostructures, it is limited to glassy materials such as polymers and amorphous metals. It has been considered infeasible for nanoimprinting crystalline metals below their melting temperatures because of the limitations on formability originating from size effect in plasticity and grain size effect. 
       SUMMARY OF THE INVENTION 
       [0006]    In view of the above-described problems, it is one objective of the invention to provide a thermoplastic imprinting method to successfully prepare metallic micro-nanostructures with sizes smaller than the grain size of bulk metals at a temperature below the melting points, i.e., firstly heating a metal, then applying load to press the heated metal into a micro-nanostructure mold to form a composite structure of the mold and the metal, and then removing the mold in the composite structure to form the replicated metallic micro-nanostructures. 
         [0007]    To achieve the above objective, in accordance with one embodiment of the invention, there is provided a method for preparing metallic micro-nanostructures comprising:
       1) heating a metal and a micro-nanostructure mold to a temperature T, where the metal is a pure metal or an alloy thereof selected from indium (In), germanium (Ge), tin (Sn), bismuth (Bi), lead (Pb), zinc (Zn), aluminum (Al), copper (Cu), gold (Au), silver (Ag), platinum (Pt) and palladium (Pd), T is greater than or equal to 0.5 T m  and less than T m , with T representing an absolute temperature and T m  representing a melting point temperature of the metal at absolute temperature scale;   2) applying load to press the metal at the temperature T into the mold to obtain a composite structure comprising the mold and the metal; and   3) removing the mold, to obtain the replicated metallic micro-nanostructures;       
 
         [0011]    In a class of this embodiment, the characteristic scale of the micro-nanostructures is 1 nm to 100 nm. 
         [0012]    In a class of this embodiment, the characteristic scale of the micro-nanostructures is 100 nm to 50 μm. 
         [0013]    In a class of this embodiment, the micro-nanostructure mold is made from silicon, silicon oxide, aluminum oxide or other inorganic oxides. 
         [0014]    In a class of this embodiment, the way of removing mold in step 3) comprises chemical etching. 
         [0015]    In a class of this embodiment, the step 3) specifically comprises the sub-steps of: putting the composite structure comprising the mold and the metal in an alkaline solution or an acid solution, and heating the same, and carrying out soaking and washing using deionized water, and absolute ethanol in sequence after completely etching the mold away, to obtain the replicated metallic micro-nanostructures. 
         [0016]    In a class of this embodiment, the metal is bulk, flaky or granular metal. 
         [0017]    The principle of the present invention is specifically described as below. 
         [0018]    According to the present invention, significantly enhanced mobility (or diffusive motion) of atoms of a metal and defects thereamong under high temperature and pressure allows precision replicating micro-nanostructures of a mold. Without loss of generality, taking a mold with pillared nanopores for example, by denoting the nanopore size as d and the length of metal following into the pores as L, respectively, and under a constant stress loading, then the length of metallic nanopillar (L) can be generally expressed as a function of temperature T, constant stress σ, pore diameter d and time t 
         [0000]        L=f (σ, T,t,d )  (1)
 
         [0019]    By applying the typical Norton-Bailey power law relation, the following equation can be obtained 
         [0000]        L=L   0   +Aσ   n   t   m   (2)
 
         [0000]    where L 0  approximates the length of the metal flowing into the nanopores during the load increasing from zero to the constant stress σ, which is in general a function of pore diameter, temperature, constant stress and rate of loading. A is a constant depending on temperature, pore diameter, and parameters of a material, such as shear modulus. n and m are the stress and time exponents of creep rate, respectively. Therefore, for examples of preparing metallic micro-nanostructures under constant temperature and constant load, the relation between lengths of nanowires and processing time can be simplified as: 
         [0000]        L=L   0   +Bt   m   (3)
 
         [0020]    The present invention has the following advantages and beneficial effects: 
         [0021]    (1) The method has the advantages of simple steps, low costs, high yields and high precision with good controllability. 
         [0022]    (2) Micro-nanostructures prepared by using the present invention can achieve very small size and high ratios, up to 200-400. 
         [0023]    (3) Prepared metallic micro-nanostructures by the present invention have wide applications, such as the application of good electrical conductivity of nanoparticles of Ag, Cu, etc., in the field of microelectronics, application of Ag nanoparticles in medical treatment due to their strong bactericidal activity, no antibiotic resistance and excellent permeability, application of nanoparticles of Au, Ag, Cu, etc., in the fields of biochemical sensing, surface enhanced Raman spectroscopy, biomedicine, nanophotonics, etc. based on surface plasmon resonance, and application of nanomaterials of platinum family metals such as Pt, Pd and Cu and corresponding alloys, in the fields of fuel cells, petrochemical industry, etc. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The invention is described hereinbelow with reference to the accompanying drawings, in which: 
           [0025]      FIG. 1  is a diagram of a method according to the present invention, wherein 1 represents a bulk metal or bulk metal alloy, and 2 represents a micro-nanostructure mold; 
           [0026]      FIG. 2  is a flow diagram of preparing metallic micro-nanostructures according to the present invention; 
           [0027]      FIG. 3  is a view of nanowires arrays of Au prepared in Example 1 of the present invention as well as a high-resolution Transmission Electron Microscope (TEM) image of a single Au nanowire having the diameter of about 8 nm which clearly shows the lattice orientation of the prepared Au nanowire; 
           [0028]      FIG. 4  is a view of influence of processing temperatures on lengths of prepared Au nanowires; 
           [0029]      FIG. 5  is a view of influence of processing time on lengths of prepared Au nanowires; 
           [0030]      FIG. 6  is a view of nanowires arrays of Ag prepared according to the present invention; 
           [0031]      FIG. 7  is a view of nanowires arrays of Bi prepared according to the present invention; 
           [0032]      FIG. 8  is a view of nanowires arrays of Pt prepared according to the present invention; 
           [0033]      FIG. 9  is a view of nanowires arrays of Pd prepared according to the present invention; 
           [0034]      FIG. 10  is a view of nanowires arrays of metal alloy Au 80 Si 20  prepared according to the present invention; 
           [0035]      FIG. 11  is a view of nanowires arrays of metal alloy Au 74 Co 26  prepared according to the present invention; 
           [0036]      FIG. 12  is a view of nanowires arrays of metal alloy Ag 3 In prepared according to the present invention; 
           [0037]      FIG. 13  is a view of nanowires arrays of metal alloy Ag 55 Al 45  prepared according to the present invention; 
           [0038]      FIG. 14  is a view of nanowires arrays of metal alloy Ag 75 Ge 25  prepared according to the present invention; 
           [0039]      FIG. 15  is a view of nanowires arrays of metal alloy Cu 34.7 Zn 3.0 Sn 62.3  prepared according to the present invention; 
           [0040]      FIG. 16  is a view of nanowires arrays of copper wire prepared according to the present invention; 
           [0041]      FIGS. 17A and 17B  are a view of hierarchical micro-nanostructures of Ag prepared according to the present invention;  FIG. 17A  is a view of the hierarchical micro-nanostructures observed at low magnification, and  FIG. 17B  is a zoom-in view of the hierarchical micro-nanostructures in  FIG. 17A ; 
           [0042]      FIGS. 18A and 18B  are a view of hierarchical nanostructures of Au prepared according to the present invention;  FIG. 18A  is a view of the hierarchical nanostructures observed at low magnification, and  FIG. 18B  is a zoom-in view of the hierarchical nanostructures; and 
           [0043]      FIG. 19  illustrates the comparison of Raman spectrograms of Crystal Violet (CV) molecules placed on the surfaces of the nanowires arrays of Au prepared in the present invention and a reference bulk Au without nanowires: (a) shows Raman spectrogram of the CV molecules on the surface of the bulk Au without nanostructure, and (b), (c) and (d) represent significant enhancement effects of the nanowires arrays of Au having the diameters of nanowires of 200 nm, 90 nm and 35 nm on Raman spectrograms of the CV molecules, respectively. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0044]    For further illustrating the invention, experiments detailing a method for preparing metallic micro-nanostructures are described below. It should be noted that the following examples are intended to describe and not to limit the invention. 
       Example 1 
       [0045]    1) 43 mg of bulk Au was weighed and put on a porous anodic aluminum oxide mold (hereinafter referred to as AAO mold: purchased from Hefei Pu-Yuan Nanotechnology limited company) having a through-pore diameter of about 20 nm. 
         [0046]    2) Two plates placed oppositely, which were connected to a device capable of applying load (two plates were connected to upper and lower clamps of a universal testing machine, respectively, in all examples of the present invention), were heated by means of a high-temperature furnace. The target temperature for the plates was set to 940 K (the temperature of the plates was monitored and controlled by means of a temperature control system of the high-temperature furnace). 
         [0047]    3) The stacked bulk Au and AAO mold was interposed between the two plates after the temperature of the plates reached a predetermined temperature and stayed stable; after the bulk Au was heated, the two plates were controlled by control software of the universal testing machine to move relatively at a constant rate of displacement loading of 0.5 mm/min; and the load was removed completely at a rate of 14.4 mm/min after it reached 20 KN. 
         [0048]    4) The sample was taken out; the AAO mold was etched away by using 1-3 mol/L acid or alkaline aqueous solution (unless otherwise specified, potassium hydroxide (KOH) aqueous solution was used in all examples of the present invention) under the condition of a temperature ranging from room temperature to 100 DEG. C; and the sample was soaked and washed using deionized water, and acetone or absolute ethanol in sequence after the AAO was etched completely (unless otherwise specified, concentrations and corrosion temperatures within the above-mentioned ranges were selected in the following examples for the purpose of complete removing of the AAO mold or silicon mold, with the same process steps being selected to wash samples). 
         [0049]    5) A Scanning Electron Microscope (SEM) was used to observe the surface of the washed and dried sample, and nanowires arrays ( FIG. 3 ) on the surface of Au can be seen clearly. 
       Example 2 
       [0050]    In order to study the influences of a key processing parameter-holding time in the present invention, in this example, five pieces of Au samples with the same weight were weighed, and except for different holding times under the same constant load, all the other processing conditions for the five samples were kept the same for the purpose of investigating the effect of holding time on the lengths of the prepared Au nanowires. The specific processing steps shown as follows: 
         [0051]    1) 61.8±0.5 mg of bulk Au were weighed and put on AAO mold having a through-pore diameter of about 200 nm. 
         [0052]    2) The target temperature for plates was set to 773K. The stacked bulk Au/AAO mold was interposed between two plates after the temperature of the plates reached a predetermined temperature. 
         [0053]    3) After bulk Au was heated, the two plates were controlled by the control software of the universal testing machine to move relatively at a constant rate of force loading of 1 KN/s; after the load reached 10 KN, the five samples were held for different times under the action of this load, and then the load was removed completely at a rate of 10 KN/s. 
         [0054]    4) The samples were taken out and put in KOH aqueous solution with AAO being etched completely at a temperature of 85 DEG. C, and then soaked and washed using deionized water, and absolute ethanol in sequence. 
         [0055]    5) Lengths of nanowires in the central areas of the samples were observed using SEM and measured. 
         [0056]    Based on the experimentally measured results (black spots in  FIG. 4 ( a ) ) of the lengths of nanowires versus holding time, the above experimental data were nonlinearly fitted by applying equation (3), to obtain the following fitting result: 
         [0000]    L=1269.2+434.8×t 0.35  (full line in  FIG. 4 ( a ) ), with the unit of length for nanowires therein being nm and the unit of time being s. In addition, by defining an apparent strain rate as 
         [0000]    
       
         
           
             
               
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         [0000]    the apparent strain rates ( FIG. 4 ( b ) ) versus holding time can be obtained based on the above fitting result. It is thus clear that the apparent strain rates in this example have the magnitude of 10 −3  s −1 . This example shows that the lengths of prepared nanowires arrays can be accurately controlled by controlling the holding time. 
       Example 3 
       [0057]    In order to study the influences of another key processing parameter-hot pressing temperatures in the present invention, in this example, five Au samples with the same weight were weighed, and except for different hot pressing temperatures for the samples, all the other processing conditions for the five samples were kept the same for the purpose of investigating the effect of hot pressing temperature on the lengths of prepared Au nanowires. The specific processing steps shown as follows: 
         [0058]    1) 29.5±0.5 mg of bulk Au were weighed and pre-pressed to obtain five flat discs with thickness of 0.35±0.05 mm at a temperature of about 688 K; and then the pre-pressed Au discs were put on AAO mold having through-pore diameters of about 100 nm. 
         [0059]    2) Different target temperatures for the plates were set with respect to the different five samples, and each stacked Au/AAO mold was interposed between two plates after the temperature of the plates reached a predetermined temperature. 
         [0060]    3) After the Au disc was heated, the two plates were controlled by the control software of the universal testing machine to move relatively at a constant rate of force loading of 0.5 KN/s; after the load reached 3 KN, the samples were held for 100 s under the action of this load, and then the load was removed completely at a rate of 3 KN/s. 
         [0061]    4) The samples were taken out and put in KOH aqueous solution with the AAO being etched completely at a temperature of 85 DEG. C, and then soaked and washed using deionized water, and absolute ethanol in sequence. 
         [0062]    5) Lengths of nanowires in the central areas of the samples were observed using SEM and measured. 
         [0063]    The experimentally measured results of the lengths of nanowires versus hot pressing temperature are as shown by black spots in  FIG. 5 . It is thus clear that as the hot pressing temperature increases, the lengths of the prepared nanowires increase first, then decrease slightly and finally continuously increase within the investigated temperature range. On the whole, the trend of the lengths of the nanowires increasing along with increasing temperature can be explained by reduction of activation energy barrier for atoms (or defects) due to the temperature increasing. Also, the behavior near 550 DEG. C (i.e., 823 K) indicates that the dominated creep mechanism has changed significantly. 
       Example 4 
       [0064]    1) 0.14 g of bulk Ag was weighed and put on an AAO mold having a pore diameter of about 200 nm. 
         [0065]    2) A target temperature for the plates was set to 980 K; and the stacked bulk Ag/AAO mold was interposed between two plates after the temperature of the plates reached a predetermined temperature. 
         [0066]    3) After the bulk Ag was heated, the two plates were controlled by the control software of a testing machine to move relatively at a constant rate of displacement loading of 18 mm/min; after the load reached 10 KN, the sample was held for about 65 min under the action of the load, and then the load was removed completely at a rate of 10 KN/s. 
         [0067]    4) The sample was taken out and put in KOH aqueous solution with the AAO being etched completely at the temperature of 85 DEG. C, and then soaked and washed using deionized water, and absolute ethanol in sequence. 
         [0068]    5) The morphology of the prepared nanowires in the sample was observed by using a SEM. 
         [0069]    As shown in  FIG. 6 , the AAO mold having the thickness of about 50 μm is already completely filled up with the Ag nanowires, i.e., the aspect ratio of the prepared Ag nanowires being up to 250. 
       Example 5 
       [0070]    1) 32 mg of bulk Bi was weighed and put on an AAO mold having a pore diameter of about 200 nm. 
         [0071]    2) A target temperature for the plates was set to 533 K; and the stacked bulk Bi/AAO mold was interposed between two plates after the temperature of the plates reached a predetermined temperature. 
         [0072]    3) After the bulk Bi was heated, the two plates were controlled by the control software of a testing machine to move relatively at a constant rate of displacement loading of 3 mm/min, and the load was removed completely at a rate of 8 KN/s after it reached 8 KN. 
         [0073]    4) The sample was taken out and put in KOH aqueous solution with the AAO being etched completely at a temperature of 85 DEG. C, and then soaked and washed using deionized water, and absolute ethanol in sequence. 
         [0074]    5) The morphology of the surface nanowires of the sample was observed by using a SEM. 
         [0075]    As shown in  FIG. 7 , the AAO mold having the thickness of about 50 μm is already completely filled up with the Bi nanowires, i.e., the aspect ratio of the prepared Bi nanowires being up to 250. 
       Example 6 
       [0076]    1) 60 mg of bulk Pt was weighed and put on an AAO mold having a pore diameter of about 200 nm. 
         [0077]    2) A target temperature for the plates was set to 1093 K; and the stacked bulk Pt/AAO mold was interposed between two plates after the temperature of the plates reached a predetermined temperature. 
         [0078]    3) After the bulk Pt was heated, the two plates were controlled by the control software of a testing machine to move relatively at a constant rate of displacement loading of 1.8 mm/min; after the load reached 20 KN, the sample was held for 5 min under the action of the load, and then the load was removed completely at a rate of 0.06 mm/s. 
         [0079]    4) The sample was taken out and put in KOH aqueous solution with the AAO being etched completely at a temperature of 85 DEG. C, and then soaked and washed using deionized water, and absolute ethanol in sequence. 
         [0080]    5) The morphology of the surface nanowires of the sample was observed by using a SEM. 
         [0081]    As shown in  FIG. 8 , all the Pt nanowires have regular and similar edges and surfaces, indicating that the prepared Pt nanowires are single-crystal nanowires. 
       Example 7 
       [0082]    1) 50 mg of bulk Pd was weighed and put on an AAO mold having a pore diameter of about 130 nm. 
         [0083]    2) A target temperature for plates was set to 1093 K, and the stacked bulk Pd/AAO mold was interposed between two plates after the temperature of the plates reached a predetermined temperature. 
         [0084]    3) After the bulk Pd was heated, the two plates were controlled by the control software of a testing machine to move relatively at a constant rate of displacement loading of 1.8 mm/min; after the load reached 20 KN, the sample was held for 5 min under the action of the load, and then the load was removed completely at a rate of 0.06 mm/s. 
         [0085]    4) The sample was taken out and put in KOH aqueous solution with the AAO being etched completely at a temperature of 85 DEG. C, and then soaked and washed using deionized water, and absolute ethanol in sequence. 
         [0086]    5) The morphology of the surface nanowires of the sample was observed by using a SEM. 
         [0087]    As shown in  FIG. 9 , all the Pd nanowires are not clear enough in SEM details such as edges and surfaces, possibly due to severe oxidation of Pd at a high temperature. 
         [0088]    In addition to the above examples of preparation of nanowires arrays of pure metal, examples of preparation of nanowires of metal alloys will be further shown below. 
       Example 8 
       [0089]    1) 15 mg of bulk Au 80 Si 20  (unless otherwise specified, all subscripts denote atomic percent hereinafter) was weighed and put on an AAO mold having a pore diameter of about 200 nm. 
         [0090]    2) A target temperature for plates was set to 573 K; and the stacked bulk Au 80 Si 20 /AAO mold was interposed between two plates after the temperature of the plates reaches a predetermined temperature. 
         [0091]    3) After the bulk Au 80 Si 20  was heated, the two plates were controlled by the control software of a testing machine to move relatively at a constant rate of displacement loading of 0.6 mm/min; and the load was removed completely at a rate of 10 KN/s after it reached 10 KN. 
         [0092]    4) The sample was taken out and put in KOH aqueous solution with the AAO being etched completely at a temperature of 80 DEG. C, and then soaked and washed using deionized water, and absolute ethanol in sequence. 
         [0093]    5) The morphology of the surface nanowires of the sample was observed by using a SEM ( FIG. 10 ). 
       Example 9 
       [0094]    1) 10 mg of bulk Au 74 Co 26  was weighed and put on an AAO mold having a pore diameter of about 200 nm. 
         [0095]    2) A target temperature for the plates was set to 1048 K; and the stacked bulk Au 74 Co 26 /AAO mold was interposed between two plates after the temperature of the plates reached a predetermined temperature. 
         [0096]    3) After the bulk Au 74 Co 26  was heated, the two plates were controlled by the control software of a testing machine to move relatively at a constant rate of displacement loading of 1.8 mm/min; after the load reached 10 KN, the sample was held for 2 min under this load, and then the load was removed completely at a rate of 0.12 mm/s. 
         [0097]    4) The sample was taken out and put in KOH aqueous solution with the AAO being etched completely at a temperature of 80 DEG. C, and then soaked and washed using deionized water, and absolute ethanol in sequence. 
         [0098]    5) The morphology of the surface nanowires of the sample was observed by using a SEM ( FIG. 11 ). 
       Example 10 
       [0099]    1) 25 mg of bulk Ag 3 In (which represents that the atomic ratio of Ag to In is 3:1 herein) was weighed and put on an AAO mold having a pore diameter of about 100 nm. 
         [0100]    2) A target temperature for the plates was set to 720 K, and the stacked bulk Ag 3 In/AAO mold was interposed between two plates after the temperature of the plates reached a predetermined temperature. 
         [0101]    3) After the bulk Ag 3 In was heated, the two plates were controlled by the control software of a testing machine to move relatively at a constant rate of displacement loading of 1.8 mm/min; and the load was removed completely at a rate of 10 KN/s after it reached 10 KN. 
         [0102]    4) The sample was taken out and put in KOH aqueous solution with the AAO being etched completely at a temperature of 80 DEG. C, and then soaked and washed using deionized water, and absolute ethanol in sequence. 
         [0103]    5) The morphology of the surface nanowires of the sample was observed by using a SEM ( FIG. 12 ). 
       Example 11 
       [0104]    1) 20 mg of bulk Ag 55 Al 45  was weighed and put on an AAO mold having a pore diameter of about 100 nm. 
         [0105]    2) A target temperature for the plates was set to 720 K, and the stacked bulk Ag 55 Al 45 /AAO mold was interposed between two plates after the temperature of the plates reached a predetermined temperature. 
         [0106]    3) After the bulk Ag 55 Al 45  was heated, the two plates were controlled by the control software of the testing machine to move relatively at a constant rate of displacement loading of 0.6 mm/min; and the load was removed completely at a rate of 10 KN/s after it reached 10 KN. 
         [0107]    4) The sample was taken out and put in KOH aqueous solution with the AAO being etched completely at a temperature of 80 DEG. C, and then soaked and washed using deionized water, and absolute ethanol in sequence. 
         [0108]    5) The morphology of the surface nanowires of the sample was observed by using a SEM ( FIG. 13 ). 
       Example 12 
       [0109]    1) 30 mg of bulk Ag 75 Ge 25  was weighed and put on an AAO mold having a pore diameter of about 200 nm. 
         [0110]    2) A target temperature for the plates was set to 720 K, and the stacked bulk Ag 75 Ge 25 /AAO mold was interposed between two plates after the temperature of the plates reached a predetermined temperature. 
         [0111]    3) After the bulk Ag 75 Ge 25  was heated, the two plates were controlled by the control software of a testing machine to move relatively at a constant rate of displacement loading of 1.8 mm/min; and the load was removed completely at a rate of 5 KN/s after it reached 5 KN. 
         [0112]    4) The sample was taken out and put in KOH aqueous solution with the AAO being etched completely at a temperature of 80 DEG. C, and then soaked and washed using deionized water, and absolute ethanol in sequence. 
         [0113]    5) The morphology of the surface nanowires of the sample was observed by using a SEM ( FIG. 14 ). 
       Example 13 
       [0114]    1) 15 mg of bulk Cu 34.7 Zn 3.0 Sn 62.3  was weighed and put on an AAO mold having a pore diameter of about 200 nm. 
         [0115]    2) A target temperature for the plates was set to 768 K, and the stacked bulk Cu 34.7 Zn 3.0 Sn 62.3 /AAO mold was interposed between the two plates after the temperature of the plates reached a predetermined temperature. 
         [0116]    3) After the bulk Cu 34.7 Zn 3.0 Sn 62.3  was heated, the two plates were controlled by the control software of a testing machine to move relatively at a constant rate of displacement loading of 1.8 mm/min; after the load reached 10 KN, the sample was held for 2 min under the action of this load, and then the load was removed completely at a rate of 5 KN/s. 
         [0117]    4) The sample was taken out and put in KOH aqueous solution with the AAO being etched completely at a temperature of 60 DEG. C, and then soaked and washed using deionized water, and absolute ethanol in sequence. 
         [0118]    5) The morphology of the surface nanowires of the sample was observed by using a SEM ( FIG. 15 ). 
       Example 14 
       [0119]    1) 20 mg of bulk plain Cu conductor was weighed and put on an AAO mold having a pore diameter of about 200 nm. 
         [0120]    2) A target temperature for the plates was set to 720 K, and the stacked bulk Cu conductor/AAO mold was interposed between two plates after the temperature of the plates reached a predetermined temperature. 
         [0121]    3) After the bulk Cu conductor was heated, the two plates were controlled by the control software of a testing machine to move relatively at a constant rate of displacement loading of 1.8 mm/min; after the load reached 20 KN, the sample was held for 2 min under the action of this load, and then the load was removed completely at a rate of 5 KN/s. 
         [0122]    4) The sample was taken out and put in KOH aqueous solution with the AAO being etched completely at a temperature of 40 DEG. C, and then soaked and washed using deionized water, and absolute ethanol in sequence. 
         [0123]    5) The morphology of the surface nanowires of the sample was observed by using a SEM ( FIG. 16 ). 
         [0124]    The above are descriptions on the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, numerous improvements and modifications can also be made without departing from the principle of the present invention, and these improvements and modifications should also fall into the scope of protection of the present invention. 
       Example 15 
       [0125]    A hierarchical micro-nanostructures of Ag by using a silicon mold combined with an AAO mold are prepared through the following processing steps: 
         [0126]    1) A silicon mold having a through-pore diameter of 0.3 mm was stacked on the surface of an AAO mold having a pore diameter of 200 nm, and then 0.11 g of Ag was weighed and put on the surface of silicon mold. 
         [0127]    2) A target temperature for plates was set to 943 K, and the stacked bulk Ag/Si mold/AAO mold was interposed between two plates after the temperature of the plates reached a predetermined temperature. 
         [0128]    3) After the bulk Ag was heated, the two plates were controlled by the control software of a testing machine to move relatively at a constant rate of displacement loading of 0.1 mm/min; after the load reached 1 KN, the sample was held for 5 min under the action of this load, and then the load was removed completely at a rate of 3.6 mm/min. 
         [0129]    4) The sample was taken out and put in KOH aqueous solution with the AAO being etched completely at a temperature of 80 DEG. C, and then soaked and washed using deionized water, and absolute ethanol in sequence. 
         [0130]    5) The morphology of the surface micron-nano two-level structure of the sample was observed by using a SEM ( FIGS. 17A-17B ). 
       Example 16 
       [0131]    Similarly, a hierarchical nanostructure of a metal or metal alloy can be prepared by using a hierarchical AAO mold. 
         [0132]    1) About 40 mg of bulk Au was weighed and put on the upper surface of the hierarchical AAO mold (the AAO mold has a base pore diameter of 200 nm, but there is a thin activated layer in its upper surface, where the nanopores have a smallest diameter of about 8 nm). 
         [0133]    2) A target temperature for the plates was set to 773 K, and the stacked bulk Au/AAO composite mold was interposed between two plates after the temperature of the plates reached a predetermined temperature. 
         [0134]    3) After the bulk Au was heated, the two plates were controlled by the control software of a testing machine to move relatively at a constant rate of displacement loading of 0.05 mm/min; after the load reached 5 KN, the sample was held for about 620 s under the action of this load, and then the load was removed completely at a rate of 5 KN/s. 
         [0135]    4) The sample was taken out and put in KOH aqueous solution with the AAO being etched completely at a temperature of 80 DEG. C, and then soaked and washed using deionized water, and absolute ethanol in sequence. 
         [0136]    5) The morphology of the surface multi-level nanostructure of the sample was observed by using a SEM ( FIGS. 18A-18B ). 
         [0137]    Additionally, this example also shows that the characteristic sizes of the nanowires that can be prepared by the method in the present invention is as small as 8 nm. 
         [0138]    To show the excellent properties of the prepared micro-nanostructures of a metal (or metal alloy), in this example, prepared nanowires arrays of Au having different diameters were used as substrates, Crystal Violet (CV for short) molecules was used as probe molecules. Significant enhancement of the substrates with nanowires arrays of Au on Raman spectra of CV molecules were observed ( FIG. 19 ). The CV molecules used herein were diluted by absolute ethanol solution and have a concentration of 10 −5  mol/L. Compared with the surface of the bulk Au without surface micro-nanostructures, the Au surfaces with nanowires arrays show significant enhancements in all characteristic Raman spectral peaks of the CV molecules ( FIG. 19 ). This example clearly demonstrates the potential application of the prepare metallic nanostructures represented by Au in high-sensitivity of molecule detection. 
         [0139]    The above examples show that the method and principle of the present invention can be generally used in preparation of metallic micro-nanostructures having various physicochemical properties and functional applications, including metallic nanostructures showing surface plasmon optical properties that are represented by Au, Ag, Cu, etc., nanostructures having photocatalytic and chemical catalytic properties that are represented by Pt, Pd, Cu, etc., nanostructures having magnetic properties that are represented by Au 74 Co 26 , and nanostructures having shape memory characteristics that are represented by Cu 34.7 Zn 3.0 Sn 62.3 . 
         [0140]    Unless otherwise indicated, the numerical ranges involved in the invention include the end values. While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.