Figure 1 – Schematic of a semiconductor crystal at absolute zero where all of the electrons are bound up by holes.  (c) DE Wolf 2013.

As we have seen, the fundamental silver halide chemistry of analogue photography depends upon the physics of semiconductors.  And yet, it was developed before anyone knew about semiconductors or even uttered the word.  Indeed, in those days they didn’t need to utter the word “analogue” in relation to photography, because there was no digital photography to contrast it with.  The same is true for watches.  In the “dinosaur ages,” when I was a lad, there were just watches not analogue watches and certainly not digital watches.  Indeed, the time is fast approaching when there may be no watches except during retro-fashion fads.

But, as I promised, semiconductors will explain a lot of the technical aspects of digital photography.  To get there we need to discuss one more aspect of semiconductor physics – qualitatively I promise.

At absolute zero the valence electrons have no excess kinetic energy and are all sitting in their valence bands.  This is shown in Figure 1.  All of the positively charged holes are paired up with negatively charged electrons – very boring.  Now as we heat up the semiconductor to room temperature, some of the thermal (heat) energy gets absorbed by the electrons and they can escape to the conduction band where they are free to move.

Figure 2 – Schematic of a semiconductor crystal at room temperature where some of the electrons have escaped the lattice and are in the conductance band. There are functionally two types of charge carriers: the free electrons and the positive holes. (c) DE Wolf 2013.

Free to move means that if I attach a battery to two sides of the crystal current will flow.  Recognize, that as they flow in the current some electrons may recombine with holes.  But then new holes appear as new electrons escape and the net effect is that electrons move form the negative side of the battery to the positive side (remember that opposites attract).

But wait! Imagine that you are watching this process from  a distance.  Is it the electrons that are moving or is it the holes.  They both appear to be moving.  It’s like the sensation of being on a train and suddenly the platform appears to be moving.  Of course, we know that the electrons have escaped the crystal lattice, while the atoms (holes) have not.  But it does look like the holes are moving, and you can even calculate a speed for this motion.

The next question that we need to ask is whether there is a way of modifying the semiconductor crystal so that has more electrons or more holes?  Well, remember that whether a material is a conductor, a semiconductor, or an insulators depends on how tightly the valence electrons are bound to the positive atomic nuclei.  Remember also that the solid-state crystal acts as a unit not as a set of individual atoms.  The net-net of all of this is that if we add a small amount of a material that tends to donate electron the crystal will have more free electrons (in the conductance band).  If we add a small amount of a material that tends to bind up electrons (in the valence band) it will have more holes.  The materials added are called “doping agents.”  Semiconductors with excess negative charge (electrons) are called “n-type semiconductors.”  Semiconductors with excess positive charge (holes) are called p-type semiconductors.

The cool thing is that doping can be a very localized process, and as a result, you can build up some very complex semiconductors capable, for instance, of creating a computer and displaying your photographs on a light emitting diode display.  That will be the topic of our next technical blog.

Figure 1 – Silver halide photographic grains, from the Wikicommons and in the public domain. Originally from Plate VII from Robert James Wallace, “The Silver ‘Grain’ in Photography” by Robert James Wallace, The Astrophysical Journal, Vol. XX, No. 2, Sept. 1904, pp. 113–122, Chicago.

Today, let’s continue with our discussion of the silver halide process.  In our previous blog, we discussed the latent image, how it was made, and how you cannot see it.  We discussed how a silver ion in the silver bromide, AgBr, grain, Ag+, gained an electron and became free silver, Ag.  This process of gaining electrons in chemistry is called “reduction.”  Now, there are lots of chemicals that can contribute electrons and, yes, you guessed it, they are called “reducing agents.”  In photography they’re also called “developers.”  Usually the developers used in photography are organic compounds, but that’s not a critical point.

Now the key to all of this is that a crystal of silver bromide will not be reduced to free silver unless it already has some free silver in it.  Remember the latent image?  The free silver kind of primes the pump for the reducing agent.  There are two fancy phrases used in chemistry for this pump priming process.  We say that the free silver catalyzes the production of free silver in those grains were it already exists.  Alternatively, we say that the free silver acts as a nucleation site for the production of more free silver.

In any event, the effect of all of this is that those grains which had free silver, that is are part of the latent image, undergo reduction to produce more free silver.  In contrast those which had no free silver are unaffected by the developer.

This is great! Right?  We now have lots of dense silver grains (see Figure 1) on the parts of the emulsion, which were exposed to light.  So can we turn on the lights now and see the image?  Absolutely, not.  The problem is that there is still lots of unexposed and, therefore, still light sensitive silver bromide in the emulsion.  So there are a few more steps in the development process.

First, you need to stop the action of the developer.  While this can be done by rinsing in water, when you are looking for good control of the process, it is more typical to use an acid stop bath that stops development in its tracks.  This makes a lot of sense, since, by definition, an acid provides lots of positively charged protons ready to steal away any remaining electrons.

Then the remaining silver bromide needs to be removed. This is generally done using sodium thiosulfate, otherwise known as hypo.  This solublizes the free silver bromide out of the emulsion by converting it to bromine and silver thiosulfate ions.  These can then be safely washed away, and if you do a good job with your washing, the image stays clean for a hundred years or more.

Figure 2 – a modern color photographis film showing the different layers. 1. Film base; 2. Subbing layer; 3. Red light sensitive layer; 4. Green light sensitive layer; 5. Yellow filter; 6. Blue light sensitive layer; 7. UV Filter; 8. Protective layer; 9. Visible light. From the Wikicommons, by Voytek S under creative commons liscense.

This is pretty much all that there is to it.  But let’s take a moment to reflect.  This brilliant concept was created and refined by hundred of chemists, physicists, and engineers over the course of more than a century.  The wonderful complexity is hidden in what seems very simple.  Therein lies the magic!  The gelatin, for instance, has to allow all the chemistry to occur, allow the removal of unreacted silver bromide, but then offer some reasonable level of mechanical stability for the silver grains for a hundred years or more – and we’re doing this with what? – egg whites and animal bones.  It’s really truly marvelous.  Figure 2 shows a modern color film with all its complex layering required to produce a subtractive color image as we have discussed previously.  And remember, at the root of all of this is the basic silver halide photgraphic process.