Synthesis, simulation & spectroscopy: physical chemistry of nanocrystals

Experiments on nanocrystalline semiconductors form a wide and rapidly expanding field of research. This chapter concentrates on two very different topics within this field. In the first part, pair formation of dopant ions in nanocrystals is discussed. After a general introduction on the influence of pair formation on the luminescence properties, pair formation in nanocrystals is discussed. Due to a difference between the connectivity for sites in the bulk and at the surface, the fraction of dopant pairs depends on the crystallite size. Simulations of the statistical distribution of dopant pair states in a nanocrystal as a function of crystal structure, size and dopant concentration are presented. A closed form approximation for the results of the simulations is derived and the validity is tested. The work presented can be used to estimate dopant pair concentrations in the case of random substitution or a lower limit for the pair concentration if preferential pair formation occurs. The second part of the chapter discusses the luminescence of a single nanocrystalline semiconductor particle. The absence of inhomogeneous broadening and other ensemble averaging effects has provided exciting new insight into the luminescence and quenching mechanisms. The linewidth, blue shift and bleaching of the luminescence of single CdSe/ZnS core/shell nanocrystals are shown and discussed. Finally, potential applications of nanocrystals as luminescent labels in biological systems are presented and a few challenges for future research are discussed

Synthesis and Photoluminescence of Nanocrystalline ZnS:Mn^(2+)

The influence of the synthesis conditions on the properties of nanocrystalline ZnS:Mn2+ is discussed. Different Mn2+ precursors and different ratios of the precursor concentrations [S2-]/[Zn2+] were used. The type of Mn2+ precursor does not have an effect on the luminescence properties in the synthesis method described. On going from an excess of [Zn2+] to an excess of [S2-] during the synthesis, the particle diameter increases from 3.7 to 5.1 nm, which is reflected by a change in the luminescence properties. Photoluminescence measurements also showed the absence of the ZnS defect luminescence around 450 nm when an excess [S2-] is used during the synthesis. This effect is explained by the filling of sulfur vacancies. The ZnS luminescence is quenched with an activation energy of 62 meV, which is assigned to the detrapping of a bound hole from such a vacancy

Probabilities for dopant pair-state formation in a nanocrystal: simulations and theory

For certain dopants, luminescence measurements allow one to distin- guish between single-ion and pair-state dopant emission in a (semicon- ductor) host. In a bulk crystal the concentration of each of these dopant- states can be calculated from the dopant fraction present in the material, and is found to correlate with luminescence measurements. However, for a nanocrystalline host-lattice, these concentrations cannot be calculated due to the difference in coordination numbers for ions at the surface (a substantial fraction in nanocrystals) and in the bulk. Here simulations of dopant pair-state distributions are presented for a zincblende nanocrystal. The probability of finding at least one pair-state in the nanocrystal and the percentage of dopants forming part of a pair-state were calculated on the basis of a statistical average of 1 . 105 simulations for the same crystal size and dopant concentration. Furthermore, the distribution of nanocrystal lattice positions over the surface and the bulk of the crystal are computed from the simulations and found to agree well with a first- order theory. Finally, a closed-form approximation of the probabilities (valid in any crystal lattice) and a rigorous upper bound for the error in the approximation are discussed

Luminescence of nanocrystalline ZnSe:Mn2+

The luminescence properties of nanocrystalline ZnSe:Mn^(2+) prepared via an inorganic chemical synthesis are described. Photoluminescence spectra show distinct ZnSe and Mn^(2+) related emissions, both of which are excited via the ZnSe host lattice. The Mn^(2+) emission wavelength and the associated luminescence decay time depend on the concentration of Mn2` incorporated in the ZnSe lattice. Temperature-dependent photoluminescence spectra and photoluminescence lifetime measurements are also presented and the results are compared with those of Mn^(2+) in bulk ZnSe

Photoelectrochemical Characterization of Nanocrystalline ZnS :Mn^(2+) Layers

Measurements of the photoelectrochemical properties of nanocrystalline ZnS electrodes doped with Mn^(2+) are presented and discussed. The observation of both anodic and cathodic photocurrent is direct evidence for the nanocrystalline nature of the system. In-situ photoluminescence measurements showed stable Mn^(2+) related photoluminescence over a large potential range. Due to the unfavourable kinetics of electron and hole transfer across the interface between the nanocrystallites and solution, it is concluded that recombination accounts for most of the charge carriers generated by illumination. Breakdown of the ZnS into elementary Zn and S^(2-) in solution was also observed at negative potential. This breakdown introduces new non-radiative decay paths and is responsible for the slow luminescence decrease as a function of operating time

Synthesis, spectroscopy and simulation of doped nanocrystals

This thesis deals with the properties of semiconductor nanocrystals (ZnS or ZnSe) in the size range (diameter) of 2 nm to 10 nm. The nanocrystals under investigation are doped with the transition metal ions manganese or copper. The goal is to study photoluminescence and electroluminescence from doped ZnS and ZnSe nanocrystals, and to describe simulations and theoretical work on the distribution of dopant ions in nanocrystals. The influence of the synthesis conditions on the properties of ZnS:Mn2+ nanocrystals is described. Different precursors and different ratios of the precursor concentrations were used and interesting effects were observed that could be explaned sucesfully by the proposed luminescence mechanism. This mechanism also explains the observed temperature dependence of the emission intensities and emission profiles. The photoelectrochemical properties of the nanocrystals are presented and discussed. Both anodic and cathodic photocurrent is observed, giving direct evidence for the nanocrystalline nature of the system. Due to the unfavorable kinetics of electron and hole transfer, the photocurrent is small and most of the charge carriers generated by illumination recombine. Because of the unfavorable kinetics, the synthesis and luminescence properties of nanocrystalline ZnSe:Mn2+ and ZnSe:Cu (prepared via a high-temperature synthesis in a dry-nitrogen atmosphere) was investigated next. The dopant emissions can both be excited via the ZnSe host lattice, indicting incorporation of the dopants into the lattice. Evidence for the preferential formation of Mn2+ dopant pair-states is presented through the luminescence and lifetime measurements. Temperature-dependent photoluminescence spectra and photoluminescence lifetime measurements are also presented and the temperature dependence of the Mn2+ emission energy and bandwidth is explained by electron-phonon coupling to an optical phonon of the ZnSe host lattice. During the growth of the nanocrystals, samples are taken and the luminescence is studied as a function of the particle size. The transition between two growth-mechanisms is observed. The growth-rate of the nanocrystals is strongly dependent on the synthesis temperature. The size of the crystals influences both the ZnSe and the Cu2+ luminescence energies due to the quantum confinement, but not the Mn2+ emission due to the local nature of this transition. Temperature-dependent measurements of the luminescence intensity, lifetime and peak positions are described. The quenching of the Cu2+ related luminescence and concomitant decrease of the lifetime is related to detrapping of an electron from a Coulomb-bound electron-hole pair. Finally, electrodes of ZnSe:Cu nanocrystals on a transparent conducting substrate (indium tin oxide) were fabricated and the potential dependence of the photoluminescence of these electrodes in an aqueous electrolyte was studied. Electroluminescence of the ZnSe:Cu nanocrystalline electrodes was observed but the efficiency was likely to be low. At the end of the thesis, simulations and a mathematical theory are presented in order to calculate the probability for dopant pair-state formation in a nanocrystal. Knowing this probability is interesting because for certain dopants luminescence properties of single-ions and pair-states of dopant ions differ markedly. As it turns out, the probability for pair-state formation is size dependent due to the changing surface to volume ratio as a function of the crystal size. In the simulations, the probability of finding at least one pair-state in the nanocrystal and the concentration of ions in pair-states were calculated based on many simulations for the same crystal size and dopant concentration. Then a technical part discusses how these results can be approximated (through Stein-Chen Poisson approximation for the probability of finding at least one pair-state) and predicted (by means of an exact analytical expression for the concentration of ions in pair-states) without simulations. These results are very powerful, as they were derived for a general crystal structure and are dependent on the connectivity structure, dopant concentration and crystal size. Therefore, these results are expected to be valid for any nanocrystal and no further simulations will be required

Oxidation and annealing of thin FeTi layers covered with Pd

The hydrogen storage material FeTi has the disadvantage to lose its sorption capacity in contact with impurities such as O and H O. A possibility to overcome this problem is to coat it with an anti-corrosive layer which is permeable for hydrogen. In this study we prepared FeTi layers covered with a 4 or 20 nm thin Pd layer. We used ion beam and sputter profiling techniques, X-ray photoelectron spectrometry and scanning probe techniques to investigate the response of these bi-layers upon annealing up to 3008C in vacuum, air and 10y5 mbar O . The layered structure remains intact up to 150 °C. At 2008C in air and O , Fe and some Ti move towards the Pd surface where they form oxide regions. At higher temperatures thicker oxide regions, presumably along the Pd grains, are formed. These processes are more pronounced for the case of 4 nm Pd. A model is presented to explain the observed phenomena. We conclude that up to 1508C 4 nm of Pd is sufficient to act as a protective layer. For a temperature of 2008C, 20 nm Pd may still provide sufficient protection against oxidation

Optical and electrical doping of silicon with holmium

2 MeV holmium ions were implanted into Czochralski grown Si at a fluence of 5.5*10^14 Ho/cm^2. Some samples were co-implanted with oxygen to a concentration of (7±1)*10^19 cm^(-3). After recrystallization, strong Ho segregation to the surface is observed, which is fully suppressed by co-doping with O. After recrystallization, photoluminescence peaks are observed at 1.197, 1.96 and 2.06 lm, characteristic for the 5-I-6 --> 5-I-8 and 5-I-7 --> 5-I-8 transitions of Ho^(3+). The Ho^(3+) luminescence lifetime at 1.197 lm is 14 ms at 12 K. The luminescence intensity shows temperature quenching with an activation energy of 11 meV, both with and without O co-doping. The observed PL quenching cannot be explained by free carrier Auger quenching, but instead must be due to energy backtransfer or electron hole pair dissociation. Spreading resistance measurements indicate that Ho exhibits donor behavior, and that in the presence of O the free carrier concentration is enhanced by more than two orders of magnitude. In the O co-doped sample 20% of the Ho^(3+) was electrically active at room temperature

Waarheid in het cybernetisch tijdperk. Heidegger in gesprek met de Grieken

This doctoral thesis starts from the hypothesis that Heidegger’s way of thinking is semantic. Before discussing this hypothesis, I take a step back and ask what is required of a scientific hypothesis. It must be capable of being tested. Its explanatory powers are tested: what does it tell us about Heidegger? Does it shed new light? Does it help us to understand him better? It is tested by confronting it with its subject matter, here Heidegger’s words. Is the hypothesis corroborated by them? Or refuted? It is possible that the test necessitates some changes or adaptations of the hypothesis. In this way, the test results are fed back into the hypothesis. It is reformulated and may then be tested again as a new variant of itself. The hypothesis is fighting for its life in a hostile environment, competing with other hypotheses, struggling with the text. There is a constant need to adapt to changing circumstances, e.g. rival hypotheses or new texts that have come to light. Without explanatory success, there is no hope that the hypothesis will survive, either in adapted form or in new hypothesis developed from it. The hypothesis exists only in such adaptation and development, i.e. in replication. It is not a stable and independent thing, but a link in a chain, bearing the load of its predecessors and oriented towards it successors. The subject matter is in the same boat. Competing with other philosophical works, Heidegger’s texts fight for limited space on the presses, on the shelves of libraries, in the minds of readers. They, too, exist only as copies, as links in a chain. Their survival depends on their success, i.e. their ability to replicate. They do this very successfully. Not only did they harass their author into producing ever more of them (Heidegger had a working life of some sixty years), but they also manipulate presses the world over into replicating them again and again. Some 50 volumes of the expected 102 in the Gesamtausgabe have appeared to date, with the end nowhere in sight. Then there are the works published during Heidegger’s lifetime, translations, and of course the enormous amount of secondary literature produced each year. Every hypothesis concerning Heidegger is another copy of his texts. Including this thesis! All this makes me feel uneasy. Am I the author of a thesis or the feeding ground for the hungry replicators in Heidegger’s texts? Is my hypothesis a window of truth or a tough fighting machine looking to score? Do I exist as a thinking living being or am I a lumbering robot, clumsily imitating through my thesis my true masters, the endlessly self-replicating genes? What does it mean to have be replicator? What does it say about my life, my death, my thought, the things I encounter, the world I live in? But now I am struck by an even more unpleasant thought. How can I ever ask these questions? How can I ever think about myself, my world, my time? All my thoughts themselves exist only as replicators. My thinking always comes too late. It is already overtaken by the thing it wants to think about. It is like a circle closing. Everywhere I go I meet the same thing: the uncanny uniformity of thought and its object. How I think, how I behave, how things present themselves – it is all the same. Heidegger’s philosophy is a new way of thinking It is my hypothesis that Heidegger was confronted with this problem. His word for the uniformity of the way we speak or think and the way things exist is “cybernetics”. This word goes back tot the development of the anti-aircraft gun and other feedback systems. It also conjures up images of internet computers (cyberspace) and humanoid robots (cyborgs). But Heidegger is not concerned with the influence of these systems on our lives. He says: before we form an opinion about gene technology, before we deal with a central heating system, something has already happened: that the thing is a feedback system and that our speech is a feedback system as well! Heidegger cares not for things, but for the way things are and the way we deal with them. According to my hypothesis, Heidegger always says: if I am here and there is a thing and I go and do something with it, e.g. discuss it, then I always cross an open space through which I can reach the thing and through which the thing can reach me. What is this open space? It is the cluster of preconceptions through which the things and I approach each other. I call this space a “semantic whole”. It is a whole, because it surrounds me and the thing I talk about. It is semantic because it consists of meanings (sêmata). Heidegger calls them “words”. In the example above (my own thesis), those words are thesis, hypothesis, power, capability, effect, result, work, success, test, feedback, etc. These words define what I encounter in my surroundings, but also how I deal with them. According to Heidegger, preconceptions are not deep-rooted ideas that you have, perhaps unknowingly, until you forced to shed them as impractical and adopt a fresh stance. Why not? Because this very process of test and success is itself surrounded by a preconception: it is “cybernetic”. Exactly the same thing happens to a scientific hypothesis, or the design of a bird’s wing. Preconceptions are not things we humans “have”. Human existence itself belongs to a cluster of semantics: we exist as replicators, as do the things around us. This is all well and good, but have we not just said that it is impossible to reflect on cybernetics because any such discussion is itself cybernetic? So how can Heidegger talk about words, preconceptions and an open space? This is where the “semantics” bit of my hypothesis comes in. The words we speak can be used to explain a thing, persuade someone, inform someone, etc. Yet they also have another quality. They can reveal a whole situation, the situation in which they were spoken. For instance, when a fawn cries for its mother who is grazing nearby, this triggers a complicated power-play between fawn, mother deer and possible predator, a manipulation of muscle power by remote control. But it is also telling of a whole situation. The fawn is deserted and afraid. Heidegger, I maintain, exploits this revelatory quality of words. He twists the words in such a way that they point away from any conceptual content, rather pointing towards their semantic horizon. Semantics Our words are sêmata because they might indicate the semantic whole to which they belong. This horizon determines in advance how we discuss a subject, and also how that subject presents itself to us prior to any discussion. This is why Heidegger says that language is not ours. We do not speak, the semantic whole does. It addresses us even before we have uttered a word. Therefore, our sêmata do not belong to us, they are given to us and sometimes taken away again. This movement to and fro is, according to my hypothesis, semantics proper: the rise and fall of clusters of meaning. Heidegger’s aim, I claim, is to bring this movement to the fore. In his words, the emergence of meaning, or its absence, may be heard. In his words, language itself may be heard to speak, or maintain a silence. But Heidegger does not leave it at that. According to my hypothesis, he goes a step further. He knows that language is not there to kindly provide us the service of accessing the things, a facilitator of the coming together of the way we speak and the way things are. Instead, it comes and goes at its own will. The mutual accessibility of things and thoughts is split. Our thought is excluded from the relationship that encompasses it. Heidegger is forced to this insight by the end of traditional philosophy. This does not mean that philosophy is over; philosophical thoughts may replicate indefinitely. What it does mean is this: philosophy originated within a semantic whole; it was metaphysics, it asked and answered the “what is” question. The driver would deal with horses; the philosopher would say what a horse is. He was concerned with being and would understand it in a certain way. Being, as understood by metaphysics, is the unchangeable building block out of which fickle and manifold reality (beings) is built. Metaphysics originated within a semantic horizon of stability, structure and building. It asked questions concerning being, but did not and could not question its horizon. The latter is Heidegger’s concern. He cares not for the history of philosophy, but only for philosophy’s semantic character. Metaphysics is a sign that points to the horizon. The horizon provides the whole of meanings within which being presents itself and within which being is thought of in philosophy. It is never itself an object of thought. Nowadays there is no need to ask philosophical “what is” questions. We do not care what a horse is; all we need are workable hypotheses about it, enabling us to gain knowledge about the horse and harness it to our cart. These hypotheses are provided by zoology, not philosophy. That in itself is a sign, according to my hypothesis. Philosophy stills exists, but is no longer meaningful. In this way, philosophy still points towards a horizon, but an absent one. Untouched by our thought, a semantic whole has set. This is relevant for Heidegger. His concern is our horizon, cybernetics. Philosophy, now meaningless, says something about our horizon. In a roundabout way it tells us of the horizon’s semantic movement. This circular route is not a regrettable detour. It is unavoidable. Any direct approach is cybernetic (see below) and hence comes too late. The horizon is unapproachable; it always recedes. Alêtheia What has all this to do with alêtheia? Alêtheia is the Greek word for truth. Exactly how it should be understood is a matter of debate. Does it mean truth in the sense of the correspondence of a proposition with a state of affairs? In my thesis I argue that for Plato in the Cratylus this is not the case. Before I go into the philosophical relevance of this, I should like to point out that for Heidegger alêtheia matters because this word points towards the horizon. Alêtheia is a sêma. It defines both the way the way things appeared to the Greeks and the way they conceived of them. A change in alêtheia is not an alteration in the concept of truth, but a revolution in the way things and thoughts encounter one another. But there is more. Alêtheia, according to Heidegger, actually names the semantic movement. The word alêtheia tells of hiding (lanthanô) and coming to the fore (a-lanthanô). As such, alêtheia is not just a sêma defining how being and thought meet. It is an indication of the semantic movement and hence of the open field which allows this moment. This field never comes to the fore, and only gives itself as a given or retracted sêma. It is inaccessible. Whenever a cluster of meanings comes to the fore or sinks away, alêtheia occurs. It should be noted that no Greek ever recognized this indication of alêtheia. But it is there, in the name. The word alêtheia therefore contains a possibility of an indication of the field of semantic movement. This possibility has never been realized and never can be realized, for it is inaccessibility itself. The open field is the very inaccessibility of the inaccessible relationship between thoughts and things. The word alêtheia will therefore retain its character of possibility. When Heidegger brings this character the fore, inaccessibility itself is brought to the fore as the inaccessible. The name for that occurrence is alêtheia. To resume: Heidegger is concerned with the inaccessible horizon of cybernetics. This horizon betrays a movement. Thus it betrays its origin, the field which allows the movement. A horizon is a sêma: it is a cluster of meanings within which we and the things around us exist. It also indicates semantics proper: the give and take of such a cluster. This is why I say that the name of “Heidegger” indicates not a new philosophy, but a new way of thinking. Heidegger twists the words in such a way that they point away from their contents to the horizon and its movement. In his thought this movement comes to the fore as an indication of the inaccessable. When this happens, alêtheia occurs. This then is my hypothesis. It is tested in three chapters. I will discuss these briefly below. Chapter 1. Truth and adaequatio The first chapter explains some of Heidegger’s thoughts at length. It starts by focusing on the “way” we speak. This way, according to Heidegger, is the proposition. This word indicates the sentence as a synthesis of subject and predicate, e.g. “the road is rough”. This proposition is true if the road is rough. We also say the proposition is “correct”. Heidegger connects the word “correct” [richtig] with “direction” or “orientation” [Richtung, Ausrichtung]. A proposition can be correct or incorrect because it is oriented towards the thing it is saying something about. This orientation is possible because that thing has already come forward, even before I say something about it. Heidegger calls this coming forward “prepredicative openness” [vorpredikative Offenheit] or “truth” [Wahrheit]. Thus, all propositional truth is based on an earlier truth. By speaking of “proposition”, Heidegger is not claiming that we only speak in such sentences and never utter, for example, exclamations or questions. He reminds us that the word stems from pro-ponere, “to put forward”. The proposition is a way of putting something forward. But this “putting” betrays a technological preconception. Putting means: fixing, positing, placing. The proposition belongs to the relationship between subject and object. The subject fixes the object before him, not in the sense of tinkering with it, but in the sense of structuring its identity. It is constructed in its objectivity as a substance-cum-qualities. This construction is then repeated in the synthetic proposition. A horizon of construction labour surrounds us, the subject of propositions, and the things around us, the objects. In ancient Greek philosophy, knowledge of things is based on the truth of insight into being. What we call propositional truth is based on metaphysical truth, alêtheia, whereas Heidegger is concerned with what happened before metaphysics became metaphysics. Greek philosophy originated in an experience. It was displaced: it was turned away from the usual relation with things and turned towards an open field where this relation as such came to the fore. In the terms of my thesis, this is a confrontation with the field which allows a semantic horizon to rise. This experience can be found in Plato’s Theaetetus. However, philosophy made an effort to forget this experience. The displacement is covered up and being is conceived of within a horizon returned to its hidden state. This experience is relevant to Heidegger because we do not have it. There is nothing that prompts us to ask questions concerning our relationship with things. There is no need of a question of being. We are needless. Heidegger raises the question whether this needlessness can be brought to the fore. Chapter 2. Truth and cybernetics In this chapter, Heidegger’s thoughts concerning alêtheia are put to the test. At some stage, Heidegger claims that alêtheia never meant anything like the truth concerning being. Greek philosophy always understood alêtheia as propositional truth. I argue that Heidegger is wrong. In the Cratylus, Plato speaks of the truth of the name, using the word alêtheia. Note that Plato does not ignore, confuse or defy the traditional philosophical distinction between words which may have meaning and propositions which may be true. This distinction originates in post-Platonic philosophy. It belongs to a horizon that Plato does not share. He knows no propositions, no building blocks for propositions (words) and hence no propositional truth or verbal meaning. Plato sketches the possibility that a word may function within what we would call an interactive environment, yet at the same time show that environment. “Truth of the name” refers to the latter. The philosophical relevance is evident. Plato indicates the possibility that a word may be cybernetic and yet tell of cybernetics as a whole. He can help Heidegger with his methodological problem! Now we must temper our enthusiasm by asking whether a word can be true at all. From Carnap we learn that it cannot. Carnap shows that philosophy is based on a misconception of the status of words. When metaphysics answers a “what is” question, it wrongly presupposes that there really is such a thing as what the word names. But this is not the case. Words constitute semantic frameworks within which sciences may utter propositions about things. There are no things outside a framework to which words refer. Propositions can be true, but words cannot. Words can merely be useful in the development of such propositions. Frameworks are cybernetic: if they are not successful, they are replaced. Besides, sciences do not need philosophy to develop their frameworks; they are perfectly capable of doing so themselves. Obviously, Carnap’s criticism is fatal for philosophy and for the hope of a truth of names. Of course, the distinction between meaning and truth is a traditional philosophical one, therefore itself belonging to a horizon. Carnap fights metaphysics with metaphysics! Furthermore, I ask whether the cybernetics of frameworks does not also apply to their content. I think it does. The point is different, however. Carnap himself is surrounded by a sort of “framework”. This shows in his words. Where do words like “framework”, “efficiency”, “success” etc. (abundant in his text) come from? These words have been given to him and he accepts them without thinking. Carnap thus confirms what he sets out to refute: the truth of words. The last part of this chapter continues the criticism: cybernetics does apply to words. What consequences does this have for truth? In the age of cybernetics, subject and object are both feedback systems, living the existence of a replicator. Any gap between them is lost in indifference. Therefore, truth is no longer a matter of correspondence. It is the efficiency of a hypothesis or even a word. The name for this efficiency is fitness. That does not mean that a cybernetic proposition fits a state of affairs. It does not resemble it, as metaphysicians have thought. It does mean that the word is adapted to its surroundings and survives in them by replication. In the Cratylus, Plato reveals a truth that does not “fit”. Is it not impertinent to take Plato in a cybernetic way? Have I not just now said that Plato’s horizon differs from ours? The whole relationship between being and thought has become meaningless. One of Plato’s names for being is idea. The word refers to the unity through which many things appear as one. I argue that this word conceals a mathematical prejudice. The unity in question is reached through mathematical reduction of the manifold things in the world. In particular, a thing’s instability – i.e. its character of temporality – is reduced to a stable core. The unity is therefore a constructed unity. It is also (and for the same reason of stability) situated outside the word. I take this as an illustration of Heidegger’s thought that philosophy is surrounded by a hidden horizon. It is relevant to us because we too are surrounded by a horizon of mathematics, of calculation. Cybernetics is calculation because cybernetic things exist only as a ratio between investment and effect. (For this reason, we are no longer singled out in nature as being the animal rationale.) This horizon differs radically from Plato’s. But Heidegger raises the question of whether Plato can still speak to us. Plato may have tried to construct semantic unity in a mathematical way, but that still leaves the problem of unity, the question of how it is that we see things as one. The same point is made at the end of the first chapter. Plato thinks there is such a thing as philosophical education [paidea], i.e. learning to turn towards being and hence toward being human. Nowadays, a philosopher has nothing to teach. But perhaps it is still possible to turn away from the things towards their semantic horizon? I claim that Heidegger appeals to us to turn around. Chapter 3. Truth and alêtheia The last chapter focuses on the alpha privativum of alêtheia, more precisely on the negativity of semantics (so called “privativity”). The relationship between “putting” [stellen] and being correct is investigated in the first part of the chapter. Heidegger claims that the “essence” [Wesen] of “technology” [Technik] is “language” [Sprache]. Of course, he is not concerned with technological matters, let alone with essence in the metaphysical sense. “Putting” is a word which names both the way things are (feedback systems, from Greek tithêmi, to put) and the way we speak about them (in propositions). Hence things are installations or instruments [Einrichtungen] and our talk of them can be correct [richtig]. This whole orientation [Richtung], however, is inaccessible to our thinking. When Heidegger says that technology is demanding [Herausforderung], he does not mean that we exploit the earth or ourselves with technological means. He means that we must always speak technologically (i.e. in propositions) and encounter technological things (systems). Thus, we are kept away from something. From what? From a problem with the relationship between things and thoughts. All this “putting” and “demanding” is in truth “pursuing with oblivion” [nachstellen mit Vergessenheit]. Here, alêtheia occurs. Something is taken away from us. Can we notice it? And if so, how? These questions are addressed in the second part of the chapter. In answer, a structure of farewell [Abschied] or renunciation [Verzicht] is revealed. For example, when we say farewell to someone and the distance between us increases, the relation between us may come to the fore. In the absence of the beloved, love appears. How does it appear? In the mode of the fail [Fehl]. That word does not indicate any shortcomings. Any calculation of shortcomings already belongs to the horizon of cybernetic calculation. It refers to the distance in which a relationship may become apparent. A sêma is not present, but its absence is revealed, in a distance. Again, this is an occurrence of alêtheia. Heidegger always points out that the word alêtheia is a composite of an alpha privativum and a form of the verb lanthanô. But he does not care about the etymology, nor about the history of the use of the term. The component lanthanô indicates the oblivion surrounding us all. This is not our oblivion, but the inaccessibility of the horizon around us. At the same time, it refers to the inaccessibility itself, i.e. to the horizon’s character of movement. The horizon of philosophy is denied to us. This in turn indicates a hidden semantic field. Yet, this oblivion may be noticed, but only in the mode of the fail. This is the acomponent of alêtheia. The last part of the chapter addresses alêtheia’s character of temporality. Cybernetics is a mode of existence, hence a mode of being. Not metaphysical being, but a moving, changing being, replication. This being is presence, i.e. things and thoughts are here as feedback systems. Presence is a mode of time. The other modes, past and future, are absent. However, they can be represented, for example through historiography and planning. Heidegger points out that presence wills something. It wants control, effect, success, survival. (In the same vain, Plato’s metaphysics can be shown to want stability and unchangeability.) It is therefore against loss, confusion, death. It is against negativity. Thus, it is against time itself. For time inevitably contains negativity: the absence of past and future. And it is both these now, but not in the mode of the presence. For example: I am now my birth and death. This time is hidden in the horizon of presence. For this reason, Heidegger calls time the abyss which hides itself and gives and takes presence. In the terms of my thesis: time is the giver and taker of sêmata, the semantic field. This temporality may therefore be termed alêtheia. Conclusion. An unpleasant experience II So, does my hypothesis survive the test? I think so. The texts confirm it. The hypothesis sheds new light on Heidegger’s work. It even makes it possible to correct Heidegger in some points and to read other philosophers in such a way that they are relevant for Heidegger’s project of a new way of thinking. Obviously, this is a cybernetic process. But have I not just said that Heidegger’s way of thought is not cybernetic? (It is not outside cybernetics, but being inside, it shows its surroundings.) Here a most unpleasant thought strikes me. It is very possible to approach Heidegger cybernetically. It is possible to describe in propositions Heidegger’s nonpropositional philosophy. Heidegger’s appeal to us to turn around, away from the self evident preconceptions towards the field of semantics, falls on deaf ears. You and I stay comfortably what we are: the subject of propositions. We know Heidegger does not speak in propositions, but do not take this seriously. We are schizophrenic. This thesis is schizophrenic. Philosophically it does not matter whether the hypothesis survives testing. What matters is whether the author – and, in the best scenario, the reader – can experience his own schizophrenia. That would mean: receive an indication of my own place in the age of cybernetics.