The Modern Student laboratory: Spectroscopy Spectroscopic Analysis of Semiconductor Colloids An Ex~erimentin Materials Science for the Advanced Inorganic or ~ h i s i c aChemistry l Laboratory Robin R. Chandler, Shelli R. Bigham, and Jeffery L. coffer' Texas Christian University, Fort Wolth, TX 76129 The study of clusters, and the materials that can be derived from them, is a rapidly expanding field of interdiseiplinary scientific interest (13).At present, however, studies of solids in general and clusters in particular receive little attention, if any, in the undergraduate classroom or laboratory. Descriptions of Electronic Structure Energy Band Theoiy in Bulk Solids A traditional description of the electronic structure of bulk semiconductor phases requires the concept of energy bands. The i n h i t e l y large number of atoms in a solid have many molecular orbitals that are closely spaced in energy. Thus, a near continuum of levels is formed. This wntinuum includes both bonding and antibonding levels and covers a range of energies. For a semiconductor, there exists an occupied series of levels of highest energy (the valence band), followed by a finite energy gap between this level and a wrresponding series of unoccupied levels, known a s the conduction band (Fig. 1).The magnitude of this energy gap (bandgap, E,) for a bulk solid is analogous to the HOMO-LUMO separation for a small molecule. Il-VI bulk crystal Il-VI cluster
Quantum Size Effectsin Clusters
A
in However, for small semiconductor clusters (20-60 diameter) this description must be altered. With decreasing cluster diameter two efffects are seen. (See Fig. 1.) The energy Levels comprising a band bemme more discrete and quantized, that is, more "molecule-like" (4). The magnitude of the bandgap energy of the clusters (E;) inmases.
This evolution of cluster properties with size has been cited as an example of so-called "quantum size effects" (4). Such effects can be observed experimentally a s a shiR in the absorption edge of a colloidal solution of the clusters relative to its bulk value: 2.4 eV ( 520 nm) for CdS 0.4 eV b3000 nm) for PbS
The intrinsic bandgap energy (E,) of a II-VI or N-VI semiconductor, whether it is a cluster or bulk material, also varies according to the nature of the metal and chalcogenide substituents. Studies Using Small Clusters In response to the lack of available experiments for these solid-state concepts, we have prepared a simple experiment that illustrates two important points concerning quantum-confined (Q-size) semiconductor clusters. The bangap of the semiconductor cluster is affected by Decreasing the average particle size of the semiconductor cluster (carried out in solutions of reverse micelles) Changing the metal in a cluster of a II-VI or IV-VI semieonductar (ZnS, CdS, PbS) (carried out in solutions of DNA) Both effects are evaluated using W-vis spectroscopy. Preparation of Stabilizing Media
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Because these clusters favor agglomeration and subsequent precipitation as bulk material, they must be kept from coming into contact with one another. This is often accomplished by preparing the clusters in stabilizing media, such as inverse micelles, complex macromolecules, gelatins, glasses, and a variety of other materials (5). HOMO
Figure 1. Energy diagram illustrating the evolution of discrete energy levels and larger bandgap for a semiconductor cluster relative to the bulk solid.
Inverse Mzcelle Solutions Thc first point ofthis study is illustrated by the preparaThe tion of O-CdS clusters in inverse micellc solutions (6,. inverse micelles are created by dissolving a surfadant in (Continued on next page) Author to whom correspondence sho.uld be addressed.
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Volume 70 Number I January 1993
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The Modern Student laborcltory: Spectroscopy
*< a
I-,
Figure 2. Schematic representation of inverse micelles, with and without CdS cluster material. heptane, then adding a small amount of water (Fig. 2). The polar head groups of the surfactant encapsulate the water into small pools, and the nonpolar tails of the surfadant extend into the hydrocarbon solution. This system is ideal for making Q-size clusters because the clusters form in the water pools where they are protected from agglomeration and precipitation. Using inverse micelle stabilizers, students can easily vary the water pool diameter by changing the amount of water added to the heptane solution. This in turn allows them to prepare clusters of varying diameter. In the f r s t part of the experiment, the student prepares two micelle solutions with different water pool sizes. The molar ratio of water to surfactant is w =
[Hz01 [surfaetantl
Inverse micelle solutions with w = 5 and w = 10 are used to stabilize the formation of Q-CdS clusters. Aqueous solutions of cadmium nitrate a i d sodium sulfidi are added successively to the inverse micelle solution, and yellow QCdS solutions are obtained almost immediately. W-vis absorption spectroscopy is then used to determine absorption edge values for the two solutions. Solutions of Highly Polymerized DNA In the second part of the experiment, highly polymerized DNA is used to stabilize Q-clusters of ZnS, CdS (71,and PbS. This unique approach to stabilization yields highly stable material and gives the student an opportunity to work with an interesting biopolymer. The student prepares three aqueous solutions of herring sperm DNA, followed by addition of a n aqueous solution of the necessary metal salt. After the addition of sodium sulfide to this solution to form the metal sulfide cluster, the resulting absorption edge values are then estimated spectroscopically. Experimental: Determining the UV-vis Absorption Edge of the Colloid Solutions Caution Although the sodmm sullidr ~olutlonswed in thm exprrm~entare not parueularly dangerous. student* ~huuld uae a laboratory h o d nnd avond splllmg the 1 M solutron due ta its odor.
Disposal:Both sections of the experiment require dilute solutions of heavy metals, which should be disposed of in a labeled waste-metal container. A dilute bleach solution will
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Journal of Chemical Education
Wavelength (nrn) F ~ g ~3r eThe enect of changmg the nverse rn cel e aiarneter on the absorpt on wge of quantm con1 nea cadmum sulf~oe casters (a) Absarpton eage 1420nm, of c mers prepared n w = 5 nverse m cel es ,w = [H,O] [AOT) (oj Aosorptlon edge (450 nmj of cl~sters prepared in w = 10 inverse micelles. neutralize any sulfidespill as well as oxidize the unused sulfide solution upon completion of the experiment. Equipment
The W-vis absorption spectra were recorded using a H P 8452A diode array spectrophotometer with quartz, l-cm-pathlength cuvettes. The Effect of the Q-CdSParticle Size
Materials AOT surfadant: his@-ethylhexy1)sulfasuccinatesodium salt (Aldrich);F W = 444.55 gtmol heptane (Baxter) Cd(NO& .4HzO (Aldrich) Na2S(Aldrich) Preparing the Inverse Micelle Solution Inverse micelle solutions are prepared to have w = 5 and w = 10.
Dissolve 1.665 g of AOT surfactant in 65 mL of heptane. Then distilled water (338 ULfor w = 5: 676 uL for w = 10) is added to the heptane soiution using a m i c h t e r syringe while the solution is stirred vigon~uslv.The inverse mlcelle solution is further stirred until it appears homogeneous. Then it is purged with nitrogen to remove oxygen from the solution. An aliquot of the micelle solution is placed in a round-bottom flask fitted with a rubber septum and purged again with nitrogen. Cluster self-assemblvis somewhat de~endenton the size of the reaction flask, h e stirring rate, and the quantity of colloidal solution being prepared. Therefore, the most reproducible results are obtained using the same flask size and stir plate settings - for equal auantities of solution (see below).
Forming Q-CdS Clusters To make Q-CdS clusters, it is important to first add 5 FL of 1M Cd(NO& .4H20 via microliter syringe to 10 mL of the inverse micelle solution in a 50-mL round-bottom flask (fmal Cd2+concentration = 5 x 104M). Then stir vigorously until the mixture is completely homogeneous. Without such stirring, the Cd2+solution tends to stay at the bottom of the reaction flask without becoming evenly dispersed in the micelles. This results in the formation of bright yellow bulk CdS on the walls of the flask immediately after adding Na2S. Then slowly add 5 pL of 1M Na2S(Aldrich)with stirring to the mixture (final SZ concentration = 5 x lo4 MI. Finally, the solutions are allowed to stabilize for a t least 20 min. Determining the Absorption Edge Then record the UV-vis spectra for the solutions between 320 nm and 600 nm. The absorption edge for each solution can then be approximated by the intersection of two lines: a straight line extrapolated from the baseline, and a line drawn through the ascending slope of the onset of absorption. (See the arrows drawn on Fig. 3.) The Effect of the Metal in the Metal Sulfide
Materials herring sperm DNA (Sigma) Cd(N03)2.4H20(Aldrich) NazS (Aldrich) Zn(N03)2 6Hz0 Pb(NO&
..
.
Preparing DNA Solutions The student prepares a DNA solution for each semiconductor cluster to be formed (ZnS, CdS, and PbS clusters). Approximately 15 mg ofherring sperm DNA is dissolved in 5.0 mL of distilled deionized H20 in a 50-mL round-bottom flask (approximately 2.5 x lo3 M on a molar nucleotide basis after final dilution to 12 mL total). The DNA is slow to dissolve and should be allowed to stand for about 30 min. It can then be mixed thoroughly with a Pasteur pipet by drawing up a small portion and allowing it to run back down the sides of the flask. This ensures that a homogeneous solution is prepared. With the use of herring sperm DNA, slight variations in nucleotide content for a given mass of material may occur due to the heterogenity of the source. At the concentrations used, this has little effect on the properties of the resultant semiconductor cluster. Thus, as a n option, the molar concentration of nucleotide can be verified using an & value of 6600 M-' cm-' a t a n absorption maximum of 260 nm for the DNA. Forming Q-CdS, Q-ZnS, and Q-PbS Clusters To form Q-CdS clusters, 5 pL of the Cd(N03X . 4H20 solution prepared previously is diluted to 2 mL in a vial (final Cd2+concentration = 4 x lo4 MI. Then 5 FL of the 1 M Na2Sused in part 1is diluted in another vial to 5 mL (final SLconcentration = 4 x lo4 MI. The nucleotide flask is fitted with a septum and slowly purged with nitrogen about 10 to 15 min. The Cd2+and S2-solutio~~s are purged with nitrogen as well. After purging, the 2 mL of Cd2+solution is added to
-CdS - - PbS .*....ZnS
-
275
450
625
800
Wavelength (nm) Figure 4. The effectof changing the metal in a binary semiconductor MS (M = Cd, Zn. Pb) on the observed UV-vis absoytion threshold. in these samples was 1:1 at 4 x 10 M with a molar Ratio of M~+/S% nucleotide concentration of 2.5 x lo-? (a)ZZn Absorption edge at 310 nm; (b) CdS, 480 nm; (c) PbS, 650 nm. the nucleotide, and the mixture is purged with nitrogen another 5 min. Then the 5 mL of SZ solution is transferred to the reaction flask containing the nucleotide and Cd2+via syringe or pipet. To make clusters of Q-ZnS (colorless) and Q-PbS (redbrown), the same procedure is carried out using 1M solutions of either Zn(NO& 6H20or Pb(N03)z. The solutions should be allowed to stabilize for 20 min before recording their UV-vis spectra between 290 nm and 750 nm. The values of the corresponding absorption edge(s) are determined as described previously
.
Results and Discussion Using Micelles to Vary Cluster Size
This experiment illustrates two key concepts as well as some practical features regarding the chemistry of semiconductor materials. The well-established inverse micelle procedure is used to produce CdS clusters with different diameters. By simply increasing the ratio of water to AOT surfactant, a larger water pool is formed that can stabilize a sliehtlv lareer cluster. This is illustrated in Fieure 3. The absorption eige of the CdS clusters that formedin micelles in the w = 5 solution is blue-shifted to higher energy than that of the CdS clusters prepared in the w = 10 solution. One appealing aspect of this experiment is the potential for the student to generate other values of w and evaluate the resultant effect on the absorption edge. Alternatively, the instructor can assign different w values to different students. However, for CdS, once tbe average cluster diameter reaches approximately 60 A, the absorption edge value reflects the bulk value of 520 nm. It then shifts no further because the particle diameter is now on the order of the size of the excited electron-hole air (exciton)of the semiconductor. (Continued on next page) Volume 70 Number 1 January 1993 A9
The Modern Student laboratory: Spectroscopy Dependence of Semiconductor Bandgap on Cluster Size
Theoretically, this size dependence of the semiconductor bandgap can be understood most simply in terms of an expression proposed by Brns (4).
where E, is the bulk bandgap value; me and mh are the effective masses for the electron and hole, respectively; R is the radius of the semiconductor particle; and E is the bulk optical dielectric coefficient. Thus, the observed shift from the bulk bandgap value is a balance between the positive kinetic energy of the system and a negative Coulombic interaction. Dependence of the Bandgap on the Nature of the Metal
This experiment illustrates another important aspect of semiconductor properties: the dependence of the bandgap on the nature of the metal and the chalcogen of a compound semiconductor. In this experiment, the effect of substituting Zn and Pb for Cd in a binary sulfide semiconductor MS is analyzed for a given, fured, meta1:sulfide ratio. M. = [SZl = 4 x Typical results are shown for [Mt21 (See Fig. 4.) The relatively large bandgap of the ZnS is reflected in an adsorption edge near 310 nm. CdS has an intermediate value near 480 nm, and PbS has the smallest bandgap of the three, near 650 nm. These observed differences in absorption threshold values can also he loosely interpreted in terms of eq 1.In this case, each semiconductor begins with a radically different bulk value (3.7 eV for ZnS: 2.4 eV. CdS: 0.4 eV. PbS) (8). Assuming comparable diameter;, the observed absorption edce is influenced most strondv bv the differences in effGctive electron masses (m*io? eiectrous and holes between the different semiconductors. This, in turn, explains why PbS is shifted the most to higher energy from its bulk value; PbS has the smallest effective mass values of the three (me* = 0.1; mh* = 0.1). This is contrasted to effective mass values of 0.310.6 for m.*lmh* of ZnS and 0.2010.50 of CdS (8). Modifications of the Experiment
There are also many ways in which to modify or expand this experiment. In the first part, students can test addi-
tional solutions with different ratios of watertosurfactant, that is, with water poolsofdiflerent sizes. In either part of the experiment, it is quite feasible for the students to use different concentrations of metal cation and sultide to test the possible range of absorption shifts Summary Through this experiment, students are exposed in a laboratory setting to the concepts of band structure in semiconductor clus&r materials.-Twofundamentally important notions are clearly illustrated:
How the energy of these bands change with particle size Why the magnitude of the bandgap is dependent upon chemical compmition Also, the undergraduate student is given the opportunity to work with a polynucleic acid, the DNA, and a microemulsion, the AOT inverse micelles. This experiment has very straightforward procedures, and it is economical in the amounts of reagents used. Increased emphasis has been placed on topics concerning solids and materials in the undergraduate curriculum. e ssemiconductor clusThis ex~erimenton the ~ r o ~ e r t i of ters can'serve a s an inte;esGng and straightforward introduction to such topics for the chemistry student. Acknowledgment We thank the NSF for the funds to purchase a diodearray spectrophotometer under the auspices of the ILI program, Grant no. USE-9151597. We also thankTexas Christian University for a contribution of matching funds. Literature Cited 1, "Clvetwand Clusb~AsswbledMatetiala"; Awrha&,R. S.;Bemhalc, J.;Ndaan,D. L., Ed% Mator Ros. Soc. Symp h. 1991,206. 2. Mptol c1uatera;Moslrwits, M., Ed.;W h y : New York,1988. 3. Mptal CluaterainPrmpiis; W e ,L., Ed.:(ACS Symp. Ser. No. 312). Amdesn ChemidSc&ty, Washishipton,DC. 1988. 4. Steiwmald.M. L.: B m . L. E.AEets. Chem. R e . 1880.23.183. 6 FM a n s r r e l l ~ n cr w e w o r d d l v r s n r d p p r r s c h r s t c rlu,lcr atabdraatmn. ace S u ~ c m a l dY . L. BNI. L E.4nn. f i t . Mare?.Srr lss9.19.471 6 hlcym,M . W a l l k g . C. K u h a r a . K.:Fcndlcr,J. J. Chem. S a Chrm C,mm 1984
.
90 7. CaITer, J. L;Chand1er.R. R. Macm Res. Sa. Symp.Pme.1881,206,521. 8. P h v e , J. I. OptimlRaeeama in &mimndmtom:Rentia-HaU: Englewmd Cliff., 1911;p 413.
Second Edition of Modern Experiments for Introductory College Chemistry The second edition of Modern Experiments for Introductory Chemistry, containing experiments from the Journal of Chemical Education that were selected because they reflect exciting innovations in lahoratory work,is available. Compiled by H. Anthony Neidig and Wilmer Stratton, the experiments are written for professors to use, not a prestyled, specific laboratory directions, but as inspiration and guide for adoption and adaptation in their individual courses. The 144-pagepaperback is $18.50 ($19.50 foreign)and can be purchased by sending a prepaid order to Subscription and B w k Order Department, Journal of Chemical Education, 1991Northampton Street, Easton, PA 18042. AU orders must he prepaid, aid the price includes the shipping.
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Journal of Chemical Education
