In the Lab

 Method:

There are several methods describe in scientific journals and books that describe the synthesis of nanowires. In this investigation, CdS will be synthesized. In order to facilitate this process, catalytic growth will be utilized. “Here the catalyst is envisioned as a nanocluster or nanodroplet that defines the diameter of and serves as a site that directs preferentially that addition of reactant to the end of growing nanowire…. ”. 2  Selected catalyst are determined by using phase diagrams that form a liquid alloy with the nanowire of interest. The phase diagram has many functions.  It serves to determine the catalyst material that will enable a liquid alloy and a solid phase nanowire to exist and the desired temperature to achieve his goal. Dr. Wang states, that this liquid alloy cluster serves as the preferential site for absorption of reactant ……and when supersaturated, the nucleation site for crystallization.”

  

                                             Figure 1.                                                            Figure 2.                                                          


Figure 1. demonstrates Au and CdS form a liquid alloy and as the concentration of CdS increases supersaturation will occur, leading to nucleation and solid CdS wires theoretically will form.

Figure 2. is a phase diagram that illustrates the thermodynamics of vapor-liquid-solid (VLS)  growth of CdS and Au that occurs around 800°C. For the synthesis of CdS nanowires, the vapor-liquid-solid (VLS) method will be used.


VLS method is the absorption of a gas phase on a solid surface which results in the growth of a crystal structure. This achieved through two phases. Phase one: evaporation of the powder precursor which is then carried through by the inert gas Ar. Phase two: the substrate, coated with the Au catalyst is used as the site of nucleation and one dimensional crystalline nanowires are formed.

 

Materials:

Quartz Tube

“Boat”/ring

CdS Precursor: Cadmium Dimethylthiocarbonate

*Process Temp = 788°C

Si Substrate: Sputter (layer of Au/Pd metal) and Colloid (colloidal gold nanoparticle)

Tube Furnace

Ar Gas/ Liquid N2

 

Prepare colloidal substrate:

 

Cut the Substrate ~5 mm wide strip, rinse with de-ionized water and blow dry with air. Apply poly-L-lysine solution to clean substrate and leave 15 minutes. The poly-Llysine aids in the adhesion of the nanoparticle gold (Au) catalyst. Blow dry with air lightly. Add colloidal gold nanoparticle solution to substrate with clean Pasteur pipette. Blow dry with air lightly.

 

 

          Figure 3. Tube furnace used for this process.                                  Figure 4. Inside the Quartz tube



 

Procedure:

 

1.      Under a chemical fume hood, place prepared substrate (sputtered) into the end of a quartz tube.

     2.      Weigh and load precursor into “boat”/ring and place into opposite end of quartz tube with a steel bolt at the other end of             tube.                                                    

3.      Place quartz tube into tube furnace.




                                                  Figure 5.  Schematic diagram of nanowire growth gas flow chamber setup.                 


4.      Vacuum air out of the system.  

5.      Maintain desired vacuum of system (sustained 30 torr for 15 minutes).

6.      After test period, set operating low pressure vacuum (~200 torr).

7.      Start Argon carrier gas flow. (Sustain 200 torr for 45 minutes).

8.      Start tube furnace. The temperature of this process is at 788°C (during this process, add liquid N2 while the furnace is heating to the desired temperature.)

9.      Once operating temperature is reached, slowly push the precursor “boat”/ring into the

furnace with a bolt moved by a magnet. After a desired growth time (~10 minutes), stop tube furnace, let cool down.

10.  Near room temperature, stop Ar flow, vent vacuum allowing system to reach atmospheric pressure, and remove quartz tube from furnace.

11.  Under a chemical  fume hood, remove substrate with grown nanowires and safely

            dispose of all hazardous materials.

12.  Repeat procedure numbers 1 thru 12 at 300 torr and 400 torr for the pressure of the carrier gas flow.

13.  Repeat procedure numbers 1 thru 12 using the colloidal substrate (colloidal gold nanoparticle) at 200 torr, 300 torr, and 400 torr.

     

RESULTS AND DISCUSSION
Characterization – Structure - Imaging – Optical Microscope


The nanowires were first examined directly on the Si substrate with optical microscope. This was to see if growth had occurred and to examine its growth along the substrate.
 
 
 
 
 
 
 
 
 
 
                 Figure 6. Optical Microscope




                                  Figure 7 and 8.
 
 
Here are images of nanowires synthesized at 200 torr on colloidal and sputtered substrates. Although the initial images offer insight into the growth of the nanostructures, more characterization is needed for further identification.
 
 
Characterization – Structure - Imaging - Electron Microscopy
 
The morphology of CdS nanowires was observed in a scanning electron microscope (SEM).

Figure 9 and 10.

 Figure 9 shows the SEM images at 200 torr of the nanowires grown on the sputtered Si substrate and figure 10 shows the nanowires grown on the colloidal substrate. On the sputtered substrate there is evidence that nice wires were synthesized, however there are many nanobelts that grow along with the nanostructures. On the colloidal substrate, we see nice dense nanowires synthesized with no real evidence that nanobelts are evident.  Colloidal substrates are often utilized if more uniform nanowires are desired.  


 


Figure 11 and 12.

 Figure 11 and 12 shows the SEM images of the nanowires grown at 300 torr sputtered and colloidal substrates respectively. On the sputtered substrate, evidence of nanowires and nanobelts are seen, however more dense region of nanowires are evident. On the colloidal substrate, the uniformity is seen once again along with a more dense region than at 200 torr of nanowires.








Figure 13 and 14.
                                                     
Figure 13 and 14 SEM images of the nanowires grown at 400 torr colloidal and sputtered respectively. On the colloidal substrate, little evidence of nanowire growth is seen. However, in the sputtered substrate, more nanobelts are seen and less dense region of nanowires. The lack of growth on the collidal substrate may be attributed to what can occur during processing when the conditions are not at optimum levels.                                                                                                                                                                                                                                               






The morphology of the CdS is also studied using the transmission electron microscope. "TEMs use electrons as "light source" and their much lower wavelength makes it possible to get a resolution a thousand times better than with a light microscope." 3


Figure 15 and 16.





 
                                       






Figure 15 shows the CdS nanowire with the Au nanoparticle at the tip.
Figure 16 shows the crystal planes in the nanowire.


Subpages (1): Conclusion
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