precision photoemission assessment.

[chaosdrone]

I was wondering what would be the most simplistic approach, i have a particular polymer composite (based on emeraldine base) and this composite should exhibit some weak photoemissive property that i wish to assess. ???

some have advised me with with an ADC that should give me up to 8 significant figures of voltage value across the polymer.

others have advised a log amp that should probe directly the interesting fraction of the value.

also an avalanche current detection assembly across the polymer was advised.

i do not really know what to do .. and even if picked one of those i would be in need for its corresponding schematic  ::)


any help would be appreciated,
and thanks.

[Alex Tsekenis]

In what sense do you want to access the polymer?

[chaosdrone]

In what sense do you want to access the polymer?

;D to assess is to estimate, characterize or evaluate a particular property

[Alex Tsekenis]

You really think I was asking for a definition?

What I meant is, do you want to scan the polymer with a probe? Do you want a quantitative measure of current or some comparison method? What source of light source will you use? Do you have a vaccum chamber?

[chaosdrone]

:-[ oh .. great in that case well:

1- I can't deduce what do you particularly mean by a probe, but i should be wet moulding the polymer beads with the composite crystals in the shape i desire or in a membrane template with the desired thickness afterwards i should have some form of conductive plates across it (but i haven't managed the details of that yet as it should be easy).

2- I should essentially be comparing between the blank and composite polymer blocks, quantitative measurements could be made afterwards to say the blank and that'd be factored by the relative values i get.

3- the source of light  should be the embedded crystals themselves, they should be emitting 2 major frequency peaks one is a fraction of an eV higher than the band gap of the polymer and the other is 1.x eV aswell higher aswell.

4. i can have access to a vacuum oven in that lab for sure but can't guarantee you high powers of available vacuum pumps .. not more than a couple HP's i think.


now.. what should I do?  :-X

[Alex Tsekenis]

I see...You could have the crystals/polymer deposited on an electrode and then form a near-uniform electric field by placing another electrode opposite to that and connect that to a high voltage source. The electrons freed will be accelerated to the anode. You can then put the assembly in a vaccum chamber and your goal would be to measure the current between the electrodes with a ridiculously sensitive and low-noise meter.

The technique with the two conductive plates sounds very familiar is that like electrophoresis used in biosciences?

Your material will be locally ionised, would it not? If you have the crystals embedded then it would be a very near uniform ionisation. I reckon you got yourself a conductor as the electrons are free to migrate. So maybe this is different to electrophoresis.

Again, it comes down to measuring tiny currents. But first we need to know what to expect from theory. I could ask my brother if this is electrophoresis in its pure form, he should know more.

[chaosdrone]

Apparently you have misunderstood me  :(

first  concerning the uniform electric field .. well thats partially right .. i need to have conductors at each end of the polymer so i can somehow measure the conductivity of the composite/polymer... so yes i'll someway or another need to implement an electric field this way.

Yet, this has got nothing to do with electrophoresis, the crystals should bear no local charge and hence they won't move in an electric field.

only the "freed electrons" though of the conducting polymer would be able to move .. and what i want to measure is how much more freed electrons got liberated by the effect of irradiation.

Besides, the crystals should not ionize as they'd not be in a soluble or in a condition favouring their ionization .. neither should i be in need for high voltages i bet ..  :-\

[Alex Tsekenis]

It is interesting how terms are misinterpreted across sciences.

Anyway, so the photoelectic effect will change the conductivity of your compound, so you should be able to measure that with the electrodes.

Is that what you are after? You mentioned 'circuit' in your first post?

[chaosdrone]

yeah ..now you understand me  ;) ....  I am after making a circuit that could measure the value of photoemission or any related value to it with 7-8 digits of precision like conductivity, avalanche effect, etc.

[Alex Tsekenis]

Do you mean decimal points? That is a lot of precision...

Have you looked for any "schientific" instruments for this purpose?

What conductivity values/range do you expect?

I dont recognise the term avalanche effect, what exactly do you mean? Maybe a process similar to avalanche breakdown in insulators/semiconductors?

[chaosdrone]

yeah, decimal points

have looked up for precision resistivity measuring devices and they provide no more than 4.5 digits display with 0.03% error, which is not good and not as portable as i wish they are very bulky indeed  :'(

the polymer's conductivity ranges from 10^-8 to 10^-11 , depends on the synthesis method ... so my values should be around that.

never mind about the avalanche effect as i have just read a paper describing the breakdown voltage of the polymer and it turned to be so high for practical thicknesses.

[Alex Tsekenis]

That is painful.

Maybe you could do this more indirectly. For example, you could look for an instrument that measures conductance G and since your material geometry will be known you could use this:

σ = (G * length) / A    [sm^-1]

For example the Fluke 289 can measure down to 1nS (1G Ohm). Try plugging in your geometry and see what sort of conductance range you might require from an instrument.

[chaosdrone]

that'd not be very accurate  ???

[Alex Tsekenis]

Well I am sorry, but there will always be a degree of inaccuracy and your synthesis method will not be any better either. You can use statistical methods to further increase accuracy/precision. At your desired resolution and signal levels noise will be dominant which translates to highly engineered instruments i.e. expensive and potentially bulkier.

The equation below can be as accurate as you want it to be, if you have accurate input you will get accurate output.

Maybe you can overdope the polymer with crystals to get a measurable output and then extrapolate based on some model from your theoryto estimate the effects of lower concentrations?

[chaosdrone]

yeah i have been reading relentlessly lately about practical evaluation of such theories like EMT, percolation, maxwell, pal model, etc... and all what i have found was an unbelievable lack in generality for such laws being very case-dependant with paradoxical behaviours for many composite cases  :o

beside the crystals should induce photoemission already in the polymer phase which puts in quite of a predicable situation having to develop my unique theoretical interpretation to whatever (supposedly very accurate) data i get.  :-[

[chaosdrone]

Well ,,apart from any quantum or coupling efficiency .... what if we assumed to have an ejected photo-electron inside a polymer ... what circuit should be most efficacious in sensing such freed electrons and what would be the minimal sensitivity of the detection (the least number of ejected electrons that could be detected)??

[chaosdrone]

i was just reading this .. http://pdfserv.maxim-ic.com/en/an/AN2236.pdf


sounding like if i succeeded in turning my very minuscule current into voltage then i'd be able to simulate that PIN diode in the circuit .. and hence gain such precision .. right?


but how :D ??

[Alex Tsekenis]

You do not have current in your circuit as current flow would be a number of charged particles (electrons here) passing through a section of your material and having a common net direction.

As I understand it, your electrons are bound in the material but not to a specific atom, much like a conductor.

My best bet with my very limited knowledge in this field would be to measure the conductivity or a similar quantity of your material. The more 'free' electrons, the higher the conductivity.

The circuit you linked is kind of the electronic equivalent of your polymer. Unless you tell me that this is the only solution I cant see how you can use this circuit with its current component values.

How about sending me some of your photoelectric goo  :P and I run some tests at uni or home? Maybe putting it on a glass substrate with two electrodes on each end.

[chaosdrone]

:D well i first got to theorize the whole thing from synthesis to advanced testing before i get granted access to the lab inwhich i'll be able to make that goo  :-[

[chaosdrone]

who would hate a test for free anyway  ;)

[Alex Tsekenis]

I hate this about academic environments. Anyway, you could claim that a preliminary goo must be produced to verify the feasibility of a method to measure the conductivity of the goo.

[chaosdrone]

:-[ yeah but even the low accuracy synthesis of a preliminary goo would require a specialized polymers lab, you know .. goo can't be prepared anywhere but in goo-labs  :(

[Alex Tsekenis]

Your toilet fits the description :P

On a different note, I don't think anyone here can readily help you with the theory behind your goo without pre-reading/research. If you can synth some goo, we can help you develop a conductivity meter.

[chaosdrone]

;D lol

well, someone advised me with measuring the shot-noise phenomenon, by the use of a current integrator.

and this idea is so appealing to me, sounds like it takes down the measurement process to the edge.
have found much circuit schematics for this though, yet i wonder which particular design should i adopt, for a simple precision current integrator?  ::)

[Alex Tsekenis]

You will have free electrons but you will not have current. You will need to accelerate them in an electrostatic field. The current integrator will integrate the tiny
current over time, thus measuring charge. If you know the time, you can work out the number of electrons freed.

The current integrator will probably take the form of an op-amp with a capacitor in the negative feedback path and a grounded non-inverting input. Your output will be -(1/C) x integral of (Iin) dt [Volts].

But you need to get those electrons moving.