Let that cook a semiconductor. Once you have succeeded when you turned aluminum spoon in the rectifier. Now the experience is no less interesting, and theoretical explanations. To put it better in chemical circle or in a school lab, And not because the experience dangerous: just at home, you will likely not have the desired substances.
First - preliminary experience. Prepare a mixture of a nitrate or acetate of lead and skip through negatives o hydrogen sulfide (work under traction!). The precipitation of lead sulfide, PbS will air dry and see how it conducts electricity. It turns out that this is the most common insulator. So being here semiconductors?
Let's not jump to conclusions and put the following, the main experience. It will have to prepare an equal number, say, 15 ml, 3%-aqueous solution of thiocarbamide NH2C(S)NH2 and 6%aqueous solution of lead acetate. Pour both solutions into a small glass. With tweezers, apply the solution on a glass plate and hold it vertically (or secure in this position). Wearing rubber gloves, pour into a glass almost full of concentrated alkali solution (caution!) and very gently stir with a glass rod, taking care not to hurt her plate. Gently heat the solution so that you have pairs; continue stirring. Ten minutes later, the glass plate carefully remove, wash under running water and dry.
And in this case you got the lead sulphide - so what's the difference?
In the second experiment the reaction proceeds slowly and the precipitate did not fall immediately. If you watched the solution, I noticed that at first it was dull and became almost as milk, and only then darkened,is intermediate compounds, decomposing, formed the black lead sulphide. And he settles on the glass in the form of a thin black film, which consists of a very small, visible only under the microscope the crystals. Therefore, the film seems very smooth, almost mirror-like.
Attach the film two electrical contact and pass current. If the lead sulphide from previous experience acted as a dielectric, and now he gives a talk! Turn on the circuit ammeter, measure the current and calculate the resistance: it will be higher than that of metals, but not so large as to constitute an obstacle to the passage of current.
Bring to the plate lit the lamp very close, and turn the current. You will find that the resistance of lead sulfide has fallen sharply. About the same will behave black film, if it's just heat. But if the light and the heat conductivity is increased, it means that we are dealing with the semiconductor!
Why the sulphide of lead such a property? We wrote the formula as PbS , but the true composition of the crystals of this substance does not quite fit it. Some compounds, among which the sulphide of lead, not subject to the law of constant composition. And they are all semiconductors. (The same, incidentally, applies to the aluminum oxide, vypryamlyayushchem AC.)
In the crystal PbS the order of the particles should, it would seem, strictly be repeated. But often due to the fact that the concentration of the solution from which the crystals obtained, vary, the order is violated. The influence of temperature and other external causes. Whatever it was, in a real crystal the ratio of sulfur atoms and the lead is not exactly 1:1. Deviations from this ratio is very small, only about 0,0005. But this is enough to properties changed significantly.
Atoms of lead and sulfur bound in the crystal two electrons: lead gives them the sulfur. Well, when the ratio of 1:1 is broken? If there's an atom of lead has no sulfur atom, the electrons will be free, they will be carriers. But such cases are not so little as it may seem. Of course, the attitude 1,0005:1 is almost equal to one, but if you remember how many atoms in a crystal, this small difference will not seem to you such trifling.
The composition of lead sulfide can be adjusted. Need it then to change its conductivity. When the sulfur atoms in the crystal becomes larger, the conductivity decreases, and when they are less, produce more free electrons, and the conductivity increases. In short, changing the ratio of atoms of sulphur and lead, it is possible to obtain the desired conductivity. This experience is difficult to put; if you do not dare to experiment, take my word that it is.
Rent a quartz tube and place it in the boat with sulphide of lead. On the other hand, enter into the tube the same boat with lead and very much heat pipe to lead began to evaporate. Sulfide in this case will absorb the pair, he will be enriched with lead, and its conductivity will increase significantly.
It only remains to answer the question, why the sulphide of lead is so sensitive to light. Light quanta reported energy to the electrons, and in each case the most effective rays with a specific wavelength. For the sulphide of lead is the infrared thermal radiation. This is why we advise you to bring the lamp closer to the film.
Incidentally, in the infrared receivers and use is usually fine semiconductor - lead sulphide.